We've had great success purchasing used iPads on eBay. Here are two collections we assembled of iPads that appear to be good deals and iPads to avoid. Note that some of the auctions might have already ended.
For examples of iPads to avoid, click here.
Some tips on having a good experience purchasing used iPads:
Find an experienced, highly rated seller
Look for a reasonable return policy
Account for shipping costs
Purchase from US-based sellers
Avoid any iPad with a replaced screen or digitizer
Avoid screen cracks that have propagated into the viewable screen. Cracks on corners are usually fine and typically not visible with an inexpensive case.
Case scratches don't matter. Case dents should be avoided as the internal components can be damaged.
Ask for a screen shot of the serial number of the unit before purchasing. The serial number is located in Settings>>About. Confirm you've received the correct device.
The device should be fully unlocked with respect to iTunes and iCloud.
A brand new iPad 5 (iPad Air 2 equivalent) is $329 (Wifi only, 32 gigabytes storage), sometimes on sale for $299 at retail locations. Keep this in mind when purchasing a used iPad. For example, a used Mini 2 (Wifi only, 16 gigabytes) in great condition should be $160 or thereabouts. A Mini 3 is a Mini 2 with the touch sensor which is a great feature, but don't overpay.
Don't purchase an iPad1/iPad2/iPad3/iPad4. The iPad Mini 1 is fine for running Takedown currently, but Apple isn't updating the Mini 1 beyond iOS 9.0. A Mini 1 should be thought of as a short-term platform.
Don't pay a large premium for an iPad with built in cellular. Though this feature is convenient, you can get the same functionality using your cellphone as a hot-spot when Wifi at the event is unreliable or unavailable. It's easy to turn a modern cellphone into a hot-spot.
Here's a link to a comparison of all the iPads (except the outrageously priced Pro versions). Click here.