What do you mean? You just answered your own question. You looked up current new market prices. Now take off a reasonable amount to take into consideration that it is used. Try selling it for $700. You might even be able to get away with $800 considering how a lot of people are.
Why would you sell
a trial for windows 7? Well, if you choose to put on windows xp, many people still like it and many others can install their own OS's so I can't see a problem with trying to sell it with windows xp.