Ever since about Ohio I’ve been seeing antique shops everywhere. In tiny towns, on the highway in the middle of nowhere, in cities… They are everywhere and I have no idea who is conducting this business. The economist in me got curious so I started formulating a few ideas to explain this market.
Internal Market
My first idea is that the market is primarily internal between the people who run these businesses. They buy and sell from each other without many people introducing new currency or product. I’m sure occasionally someone buys an antique who isn’t a dealer or sells off some of their old stuff but that is the exception, not the rule.
Personal Utility
Another idea is that this market exists as a way to produce psychological value for the traders themselves. The pleasure comes from the act of trading and collecting instead of using the product for some other productive function.
Black Market
My other thought is that these are fronts for the black market. It would kind of be a perfect front. You could have high dollar transactions for one or two products, cash could be the norm, and it would take a team of specialized agents to verify the authenticity of your product. The people who run the stores are not dependent on them to put food on the table, it is more of a hobby than a business.
I’m sure the first two are closer to the truth but as a black market advocate I hope the third one has a grain of truth also.
Wandering Oak 
Your second hypothesis is probably the closest, at least from my experiences in Southeast Ohio. Most of the sales are for collectibles such as video games, old trinkets, toys, signs, etc. Antique stores and flea markets are great for cheap tables and fixtures, but it’s a complement to Wal-Mart rather than a substitute.