[swarmee]

The main ones you'll have to deal with are:
1) Cards that are altered (trimmed or recolored to a better looking condition). If you have an altered card and you send it in for grading, they will grade it Authentic, which counts at PSA for a 0.5/10, even worse than poor. So even a beautiful looking card which has been sliced to look mint will sell for about what a Fair card would.
2) Shill bidding: where a seller, consignor, or buddy bids up the price against you to make you pay more. Similar is when a bidder scores a card for you at a new price and then asks to cancel because "my son made the bid"/"I was hacked". Four people so far are in jail from Mastro auctionhouses because of the years of shill/insider bidding they used to raise prices.
3) Sellers that think an auction ended too low, and cancel the transaction (mostly an eBay thing).
4) Buyer's premiums; pay attention to the fine print when you bid at auctionhouses. Sometimes your $100 bid will mean $140 after you add the BP and shipping costs.
5) Chargebacks: PayPal allows for like 180 days for return of goods for "Items Not As Described or Delivered", much longer than the eBay auction lasts. So I have seen some buyers written about who wait three months after the auction (and it gets deleted off the eBay servers) and then request a refund through PayPal claiming they never got it. So save your PO receipts. If you can prove the card was delivered, you will win the case.
6) Sellers who send out empty boxes to your address and then claim it was delivered and you or the post office stole it.
7) Sellers who send empty boxes to your ZIP CODE but not your house who claim it was delivered because the post office only tracks to Zip code, not your specific address.
8) And then just your run of the mill guys who will "appraise" a collection for nothing, offer 90% of that to take if off your hands, knowing full well that there are gems in there worth 10x the total offer.
9) PSA/SGC cases that are fake or cracked and the card within is replaced/counterfeit. Learn all the different flips that SGC and PSA has used, use bar code reading software to confirm the card is real. Check the serial number on their websites. Look for signs of tampering, like unusually white case edges (frosting) or "popped tabs", where the security gaps that PSA has in their holders are cloudy.