Ask the seller to download Battery Detective from Cydia and send you a screenshot of the battery cycle count.
If he's unwilling to jailbreak, then, make an arrangement with him about your doing this after purchase, do it and if it's above, say, 300-400, you return the iPad.
BTW, my iPad 2's cycle count is around 150 after a year's active, multiple-hours-a-day usage. That is, I don't think any iPad2's out there have more than 400-500 cycles. It would mean they've been used for 24 hours a day.