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.
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.