Quick answer is no, you wouldn't owe any capital gains tax on selling the inherited card for $50K if its FMV on the day the person who left it to you passed away was also $50K.
As others have already mentioned, when a person passes away, all of the assets they leave to their heirs are valued at their then current FMV as of the decedent's date of death (DOD). And regardless of what the decedent may have originally paid for the items now being left to their heirs (or what some other alternative tax basis they had in those assets may have been), that FMV at the DOD becomes the new "stepped-up" tax basis of the assets in the hands of the heirs, such as yourself. In other words, when you inherited a card that was worth $50K on the day the person that left it to you passed away, your tax basis in that card is now $50K as well, as if you had just purchased the card for $50K yourself. So, if you ever go and sell the card later on, you get to deduct that $50K "stepped-up" tax basis from the sale price you get for it to determine what your capital gain (or loss) on the sale of the card is for federal income tax purposes.
FYI, depending on where you live, there may also be state and local taxes involved, which I'm not even going to try to get into. And also know that it is possible to pick an alternative valuation date to come up with the FMV of an inherited asset, and thus its "stepped-up" tax basis for tax purposes. Not going to get into any particulars regarding what or how to do it, but just know that you can choose a valuation date of up to 6 months after the decedent's date of death. Most people just go with the (DOD) for FMV purposes though.
Also know that the sale of inherited assets by an heir is automatically afforded Long Term Capital Gain/(Loss) treatment, regardless of how long they end up owning the inherited asset before selling it. In other words, you don't have to wait a full year before selling the inherited card to get the LTCG max rate of no more than a 20% federal tax on the profit from the sale of the card.
Now, even though you may not end up with a taxable capital gain from selling the inherited card, you still should be reporting its sale, along with showing the "stepped-up" tax basis you're claiming for it, on your federal income tax return for the year in which you end up selling the card. The real question then becomes how do you prove to the IRS, should they ever come asking questions of you, what your "stepped-up" tax basis in the card is. First thing I would suggest in your case is to check and see if there was someone appointed as the executor or fiduciary of the decedent's estate, and find and ask them if there was any kind of formal or informal appraisal or valuation done to come up with a FMV for the card you inherited. And if so, see if you can get a copy of it for your own tax records, and to support the "stepped-up" tax basis you reported for the card on your income tax return. And if the decedent's estate was large enough that a federal estate tax return actually had to be filed for it, you definitely need to find the executor/fiduciary responsible for filing it, and ask them exactly what the FMV of that card is shown as on the decedent's estate tax return. You want to make 100% certain that you then use the exact same FMV for that card, as shown on the decedent' estate tax return, as the "stepped-up tax basis when you sell and report it on your personal federal income tax return.
And if by chance there was no estate tax return that needed to be filed, and no formal valuation or appraisal was done of the card by the estate executor or fiduciary, you may want to do your own formal/informal valuation or appraisal to be able to document and support how you came up with the FMV of that card you are going to use as its "stepped-up" tax basis for your own income tax purposes. You could try reaching out to someone you may know at an AH/dealer who has experience in the hobby, to see if they'd be willing to give you a written appraisal/valuation of the card in question. Absent that, maybe go online or search AH sites to see if you can find comparable sales of the same card to support the FMV you're using to determine its "stepped-up" basis. And as a sort of last resort if you can't really find any recent comps, or have anyone provide an appraisal/valuation for you, as long as you end up selling the card fairly quickly after inheriting it, you could always just argue to the IRS that not enough time has passed for the card's FMV to have changed from what it was on the date the person you inherited it from passed away. In which case you'd argue that what you ended up selling the card for set its FMV for tax basis purposes as well.
The tax code/section reference that Adam posted and referred to in the previous post actually has to do with the decedent's estate and the potential filing of a federal estate tax return, NOT with you as an heir to the estate and the selling of an inherited asset from it, and the reporting of that sale on your personal income tax return. If you want to be more technically correct, here is the accurate description and references to look at for additional info regarding the reporting of "stepped-up" tax basis from sales of inherited assets. This following is from the actual IRS website:
Basis and Recordkeeping
Basis is the amount of your investment in property for tax purposes. The basis of property you buy is usually its cost. There are special rules for certain kinds of property, such as inherited property. You need to know your basis to figure any gain or loss on the sale or other disposition of the property. You must keep accurate records that show the basis and, if applicable, adjusted basis of your property. Your records should show the purchase price, including commissions; increases to basis, such as the cost of improvements; and decreases to basis, such as depreciation, nondividend distributions on stock, and stock splits.
If you received a Schedule A to Form 8971 from an executor of an estate or other person required to file an estate tax return, you may be required to report a basis consistent with the estate tax value of the property.
For more information on consistent basis reporting and basis generally, see Column (e)—Cost or Other Basis in the Instructions for Form 8949, and the following publications.
Pub. 551, Basis of Assets.
Pub. 550, Investment Income and
Expenses.
This is a link to a copy of the ITS Form 8949:
https://www.irs.gov/pub/irs-pdf/f8949.pdf
The following is an excerpt from the IRS instructions in regard to Column (e) of Form 8949, as referenced and referred to above:
Column (e)—Cost or Other Basis
The basis of property you buy is usually its cost, including the purchase price and any costs of purchase, such as commissions. You may not be able to use the actual cost as the basis if you inherited the property, got it as a gift, or received it in a tax-free exchange or involuntary conversion or in connection with a “wash sale.” If you don't use the actual cost, attach an explanation of your basis.
The basis of property acquired by gift is generally the basis of the property in the hands of the donor. The basis of inherited property is generally the fair market value at the date of death. See Pub. 551 for details.
If you sold property that you inherited from someone who died in 2010 and the executor made the election to file Form 8939, see Pub. 4895.
If you elected to recognize gain on property held on January 1, 2001, your basis in the property is its closing market price or fair market value, whichever applies, on the date of the deemed sale and reacquisition, whether the deemed sale resulted in a gain or an unallowed loss.
Schedule A to Form 8971—Consistent basis reporting. If you received a Schedule A to Form 8971 from an executor of an estate or other person required to file an estate tax return and you are a beneficiary who receives (or is to receive) property from that estate, you will be required to report a basis consistent with the final estate tax value of the property if Part 2, column C, of the Schedule A you received indicates that the property increased the estate tax liability of the decedent. In this case, first use an amount that is equal to or less than the final estate tax value listed in Part 2, column E, of the Schedule A. This amount is your initial basis in the property. You then adjust your initial basis in the property, as described under Adjustments to basis, later. The resulting amount is entered in column (e) of Form 8949.
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This is an Image: taxtip.gifIf you received a supplemental Schedule A to Form 8971, use the most recently dated supplemental Schedule A to determine your initial basis.
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Penalties for inconsistent basis reporting. If you use an initial basis that is more than the amount listed in Part 2, column E, of the Schedule A to figure your basis in the property and Part 2, column C, of the Schedule A indicates that the property increased the estate tax liability of the decedent, you may be subject to a penalty equal to 20% of any resulting underpayment of tax because the basis reported isn’t consistent with the final estate tax value of the property.
For more details, see Pub. 551, Pub. 550, or the Instructions for Form 8971 and Schedule A, available at IRS.gov/Form8971.
Hope this helps.