I would guess Gazelle is banking on the iPad 1 stream from Apple drying up, at which point those iPads they bought for $300 are going to be easily sellable at $350+. It's not an iPad 2, but it's also not $500.
Gazelle says they resell on eBay / Amazon. Probably works out better for everyone, since who buys expensive electronics from a low experience seller on eBay?
Likely eBay. Which is why you should never sell your stuff to Gazelle. They consistently, ahem, ALWAYS underprice items.
The best tip for finding what the value of your item is to sign in to ebay (yeah, make an account). Search for your item, then click in the Advanced search options to the right and choose to view Completed Listings.
Make sure to filter by auctions only that have ended most recently. Those prices are often the best you are going to get, and you can average out similar listings to get an average price.
