Before buying a used tractor on eBay, make sure you have adequate information about the condition of the tractor. Make sure it is large enough for your intended use as well.
Buying a used tractor from a private seller rather than a dealer usually means it is going to be purchased "as is." If it needs any repairs, the buyer is responsible for them. Make sure to examine all photographs closely, and ask about critical components if information about them is not included in the listing. Buy from a seller with a good rating, and be cautious about deals that seem too good to be true, because usually they are.
Make sure the tractor meets your needs. Hilly or rough terrain may need more horsepower than a flat farm. The type of attachments might affect the decision as well. Larger plows or heavier forms of equipment generally need more horsepower. Check the hitch type to make sure it matches your equipment or can be modified to do so.
If you are buying older or antique tractors to use for work, ask about the steering and braking capabilities. Make sure it runs well even when hot. Be wary of sellers who are unwilling to answer these questions. Also make sure that replacement parts are still available for that model.