Because in some case, peer to peer only means buyer interact directly with seller. Although there are non-centralized P2P marketplace such as OpenBazaar, which unfortunately never took off.
And how the blockchain handle dispute between buyer and seller?
I have never had a chance to use OpenBazaar, so I can't really give an opinion about why it never took off.
OpenBazaar team attempt to revive it under name OpenBazaar 3.0, so you might try it in the future.
A feedback system similar to what worked for Ebay when they first launched their online marketplace would be used. This would allow other marketplace users to be warned of bad actors.
Could you explain in detail about old Ebay feedback system? I tried search about it, but most website talk about current or recent system.