Yeah I use offerup but like you said it's pretty much only active in certain areas. I've bought stuff locally on there occasionally but in my area it's not super active, it's something to check sometimes.
I am not keen about their site and app though, you can only search by zip code, not names. And you have to be careful about looking at the listing age because it's just a chronological results list, not bad in theory but when you wonder why you're getting no response from the seller then see it was posted 4 years ago...yeah.
Offerup seems to hate vpns which I've turned off begrudgingly to use in the past until nord locked me out of my account over some shenanigans on their end, which prompted me to get proton and now offerup works fine on it. Probably not a popular enough VPN for them to ban their ip addresses.
Anyway, it's just one app I check sometimes. Fb marketplace being the most active in my area unfortunately