Lots of great ideas, thanks guys.
About listing expirations, the way I would probably do it is that listings would be good for say, 3 months, and then you'd get notified over email that the listing would be "archived" one week before the end of the 3 months. You could then click a button on your listing to say "keep this open for another 3 months". If you didn't do that, then it would be "archived" (i.e. no longer searchable or available for sale) -- but you could always unarchive it at any point within your accout.
I tend to view this as a sort of classifieds site, instead of what Ebay or Etsy does (so more like Craigslist, but definitely not as barebones as that is). So a seller can list the types of payments that they take, and a buyer contacts the seller to purchase something -- and all money goes between the two of them. That relieves me of the legal and monetary aspects of it.
As far as fees, I had been debating how to handle those, and I think that probably I wouldn't charge any fees. I rent my own server (that's in place for a couple of other sites that I run), and I have space and bandwidth to hold another site, easily. What I would probably do is say that if people would be kind enough to send a donation if something of theirs sold to help cover my server fees, then I certainly wouldn't say no to that, but there's no requirement to do so (if the site becomes some huge behemoth with tons of visitors, then I might have to change that policy, of course). Does that seem reasonable?