Only bid when you know exactly what you're getting. Add shipping costs. When you decide your maximum bid for an item, include postage and handling. The added costs can take the joy out of a bargain. Be a snoop.
Many auction sites offer profiles of sellers.
Some sites identify reputable dealers, other sites block underhanded ones. Know your rights. Be sure you understand the rules and legal policies of any auction site you frequent. That way you'll know what to do if a problem arises.