There are several important factors to consider when buying an ATV on eBay.
First and most important is to find out who made it.
Then research that manufacture.
Find out if they have a distribution center here in the states.
Are they online, can you call someone, do they have parts?
Most reputable Chinese quad manufactures and distributors forbid the sale of their product on eBay.

Then there are all the rest that flood eBay with cheap quads.
Most of these are being sold by someone who bought a few to try and will be gone when he realizes these things need parts and he cant get them.
Where does that leave you?
Hung out to dry.

The long and short of it is do your homework, research the manufacture and buy only quads you can get parts for because these quads will break (usually shortly after you start it).