it depends. if its a private seller, he has no obligation to guarantee it will work. Most online vendors have either a "start up" warantee (guarantees that the motor will start when you get it all put together) or a 30 day warantee, so if anything breaks w/in 30 days, they'll replace the part.

Since its a private seller, you'll just have to ask him questions and try and judge wether he's trustworthy.

some good questions would be:

what car is it from (obviously)?
milage?
why is it out of the car? (accident, what kind of accident, etc)
compression test numbers?
leakdown test?