For better or for worse, you are allowed to sell item for whatever price you want, and if someone doesn't like it, they should ignore it. Trash talking, putting down, "crapping on" or insulting other people's trades can be considered abuse. You can report people for doing this by basically saying this when submitting your ticket:

"Summary: This post is abusive towards another user.
Reason: Trash talking my trade, posting unwanted price checks."