After a completed purchase, you can leave feedback about the other party. Feedback helps other members see who they are trading with, which is why it must be honest. There are also limits on what you may write in feedback.
Not allowed
Collecting fake feedback and ratings via your own account or the accounts of people close to you, for example to get around restrictions or to appear more trustworthy as a seller
Making fake purchases to lower a seller's feedback score
Pressuring the other party to leave certain feedback in exchange for solving a problem
Personal or contact details
Hateful, racist or offensive statements
Links
Advertising for competing businesses
Allowed
Feedback where the buyer praises the seller for fast delivery
Negative feedback where the seller writes that the buyer never paid