How can I prevent the users from feeling awkward or uncomfortable when talking to an unknown user with no available profile or references at all?

I'm analysing a web project where you can connect and talk to other users, but there's no information available about them but their usernames. This site is intended to help people to communicate with other persons to find solutions to their needs of any kind. Anonymity is a core requirement. Privacy and trust are up to the users.

Imagine that you want to get a bike, and another user sells one, but you don't know his/her real name, how does he/she look like or any other reference. Why does this feel awkward or uncomfortable? What psychological elements can explain this?

Would be a good idea to tell users in the first term that anonymity is vital and anything else is up to them? Would that change their perception and dismiss the uncomfortable feeling?