Carfax states on its website that it specializes only in cars and light trucks. However, some motorcycles are on Carfax. The database does not cover all VIN numbers, though, so a motorcycle not being found on Carfax does not mean that the seller is not legitimate.
If you do find a motorcycle on Carfax, you should be able to purchase a history report on it. Reports are $39.99 for one, $59.99 for three, or $99.99 for six. A Carfax report includes a detailed ownership history, title history broken down by owner, any additional history such as structural damage or accidents reported, and a detailed history of vehicle service, title updates, and damage reports. Carfax reports are extremely thorough and a great resource in determining the quality of a vehicle, but again, few motorcycles are included in their VIN database.