One thing to remember is that by selling on Amazon Marketplace, you are required to accept returns if the buyer for some reason decides to do so (i.e. hears about the new iPad 3 and decides to wait for it). Then you are left waiting for the iPad to be sent back to you before you can then resell all the while the value is going down due to the hype of the new iPad being released. Most of the consumer populous doesn't read macrumors or follow the rumors so they have no idea a new version is on the horizon.
I do believe now is the time to sell iPads because once the iPad 3 is officially announced the values will drop like a stone (especially if Apple lowers the price of a brand new iPad 2 to something like $300-400 because who is going to pay that for a used one?).
BTW - I sold my 64gb iPad 1 for $350 on Craigslist last week - normally I do sell on ebay or Amazon, but between the fees and the above scenario I decided to do craigslist even though I do dislike it much more. Cash in hand and no returns