IMO, a key rule... never bid against yourself. It wont get you very far in business. Hence, if you have a listed price, don't say another price, let the other person make a bid. Even if very very low you will gain information from them making an offer and sometimes it will be higher then you expect. Maybe even higher then you would have asked. Second key rule, in general (might be slightly different with cheap ass bottom feeding ebay buyers).... most people undervalue what they have and overvalue what they want.
This is generally true. Think about real estate transactions. Each side feels like the other is in the power position. Those selling their house think that the first person making an offer is the only person who will be interested in their house, so feel compelled to accept the offer lest they not receive another, whereas buyers who like a house will think every other buyer will love the same house so feel compelled to move quickly and with strong offers.
Clearly as a generality it does not always hold in specific cases, but is well worth remembering during the process.