there's a cost to anonymity, and one of them is people like to check buyers profiles to see if they are rmeotely serious buyers or not.
user to user selling sites are already packed with low ballers, bots and fake accounts. some people would like to not even interact with them.
anonymity is also attracts fake listings more. not that fbm doesnt have them but its usually very obvious when a listing is fake, if not, the profile its attached to.