You have only a couple of options
1) Attempt to reestablish contact with the previous owner and ask them to remove the iPad from their account so that it can be unlocked for activation.
2) Visit https://www.icloud.com/activationlock/ and enter the IMEI or serial number in order to determine if the device has been stolen. If the iPad has been stolen, then there is nothing that can be done for you.
In the future, if you wish to save money on your Apple hardware purchases, and you wish to use a third-party, I highly recommend purchasing your hardware through a reputable dealer such as an authorized Apple reseller. Alternatively, there are reputable sites such as New Egg or Amazon that also feature product listings from third parties, but such listings often come with consumer protections so that in the event of a suspicious transaction, the buyer is able to recoup their money.
The safest, and best route is to visit Apple's own online store for certified refurbished stock. Products sold through Apple's certified refurbished program come with a 1 year warranty, just like new Apple products. Here is the link to the Certified Refurbished store.
http://www.apple.com/shop/browse/home/specialdeals/mac
Best of luck to you