So someone is only entitles to sell 1 copy of their work because once they do that it's public domain? Yeah, that's not going to work.

You may have an expectation of how a digital marketplace works, but it's up to you to understand how it works before putting down your money. I think these markets should also make their terms readily accessible and understandable, but the point stands that the seller can set terms of the transaction. It's then up to the buyers whether they accept them or not.