Building reputation always takes time. Sending payments to strangers online needs a leap of faith. I've done that many many times and fortunatelly the times I've been scammed were very few and involved slightly deffective items, not total rip offs.

So I think the reputation thing is related to the npub itself and its history. But you never really know.

Maybe Nostr doesn't need a special marketplace app, maybe it just needs some kind of auction widget (similar to the lightning invoice, just interactive). Dunno. But probably it's just that people just don't think there is a big enough group of people on nostr for it to be an attractive place for commerce?