My mother used to say (probably still says) "If you can't say something nice, don't say anything at all".
This is a community, with members supporting each other in many ways.
If I was selling something I would like people to say "GLWS" or "those keycaps look really nice, wish I had some spare $$$".
To me that shows respect for the seller, compliments the stuff being sold, and just makes everyone happy.
If someone has a problem with a particular sale, they can PM the seller, report to a mod, or even, if they absolutely must, start a new thread in off topic to have a whinge.