If you'd rather not deal with the hassles of flakey Craigslist and eBay buyers, you can easily send in your old iPad to Gazelle for a considerable trade-in value. The recommerce site is offering the follow prices for each current iPad model, available after the cut.
[Update] Gazelle has announced that because of Apple's own price reductions ($399 for a first-generation iPad) and trade-in volume, the site is not offering the same pricing plans it promised this morning. If you're locked in already you're safe, but if you haven't done so you might be better off selling it somewhere else. Maybe set up a table outside your local grocery store next to the Girl Scouts selling their cookies?