[Directly]

Three factors- Location, Overhead, Advertising.

First--Try attending some shows or setting up some shows, You might even try a flea market booth, pay maybe 100.00 month. Advantage you have other venders selling all kinds of other items which attracts all kinds of shoppers --instant traffic! I owed a shop and the building in a busy location for twenty years but wasn't needed for any income. Yes we did great before eBay, especially during the beanie baby graze! Was direct with Topps, Upper Deck, Fleer and Wizards of the Coast. I've heard product is difficult to get in any volume to make any money so you would have to diversity, one advantage of a location sellers bringing in items to sell--
A 52 Topps mantle, 52 Topps and 48 leaf Robinson, several Cobbs, Jordan RC came through in our door plus too many items to list --but again was before the internet--

Having a shop for income or fun are two questions you need to ask yourself.--you can always try a shop, get a retail tax licenses, sign a year lease if required at the most, mark out any extra clauses in the contract that requires paying any thing extra, Property taxes, Equipment repairs, etc.-so be diligent with your homework and the decision is yours alone--good luck!