ok. question is not about ui/mobile/web anything. it's how will you break chicken and egg. what's the creative marketing scheme to get that done? usually it involves cracking one side first vs. the other. in the used / deep discount space that typically means supply where supply siders are also users on the demand side. in b2c it means demand first. i know local commerce like no one's business.
My idea is to bring the buyers first. Doing thing that doesn't scale the first weeks like inviting gamers personally and give free credits to grow the inventory. But I'd love to hear what do you suggest.
I like your idea of letting buyers express interest in things that aren't actually for sale yet, which helps sellers gauge what demand there is. But you do need to follow up quickly with sellers or the buyers will end up going elsewhere. Perhaps you could add something unique e.g. paying with Bitcoin?