You're not considering FB Marketplace ... the primary interaction there is via FB Messenger. I can foresee the following:
1. see something you like
2. message seller and arrange meetup/pickup
3. upon inspection, pay via messenger
4. transaction done
1. see something you like
2. message seller and arrange meetup/pickup
3. upon inspection, pay via messenger
4. transaction done
Unlike other communication that is more ephemeral, it is very likely that any monetary transaction will be permanently and irrevocably attached to the facebook identity.