I don't think anything extra needs to looked at since iPads are standardized to be pretty much the same. I got a used one from craigslist for a really good price. ($300 for 32gb gen 1) I'm lucky because the guy repaired apple products for a living. he threw in the apple case and all the cords extra. most other listings I saw threw in cases and extras too...I don't think I can say the same about eBay. it's also safe to assume most used iPads will be in good condition since most people take care of it and put it in a case immediately after purchase.
but I think you can ask these two questions to get feel for the seller:
1. what firmware is it on?
2. when did you buy it?
...if you're lucky, you'll get an iPad bought as a Christmas present, so it might still be under AppleCare for a couple more months.