I have sadly come around to thinking that online "sell stuff" websites are basically doomed to failure. Marketplace now owns the space, but boy howdy, Craigslist really dropped the ball. Craigslist, as near as I can tell, has never once moved a muscle to fight scammers on their site. It straight-up drove the big crowds off. You want to buy or sell something on Craigslist now, you're dealing with 100 scammers for every 1 legitimate user. But I really like a lot of things about Craigslist: local versions of the site for every metro area and free to use for buying and only charges for selling big shit or jobs. That seems like a decent monetization scheme.

OfferUp and their ilk came in and proposed to fix the problems Craigslist wouldn't, but clearly making money was their focus, not a good customer experience. They force you to use their shit app to post listings, they serve up shit search results like Amazon for things you don't want and didn't ask for. They serve ads for shit you don't need and they also allow users to post dead listings for years on end - all to pump up their numbers and turn a profit while rendering the site about half useless.

I had my SO search for things on Marketplace too and I remember being very frustrated by how shit it was, but I can't remember what it was that was so terrible. Something about their lack of filtering making it so you had to search through gobs of crap to find things.

Anyway I wish you luck, but the secret sauce for a successful site like this seems to be elusive.