Most auction sites have some sort of user feedback rating system.
By spending some time reading the feedback information and ratings, you can get a better grasp of users and their past history on the site. If you're a buyer, it's a good idea to check out a seller's other current auctions. Lastly, if you still have questions or concerns, it's OK to email the seller. In addition, some sites allow users retract or edit feedback. With that in mind, you can try to make amends with the user and ask if he or she would retract or edit the negative.