Do you know how to tell a good engine from a bad one? How about knowing if it really has the mods he claims?

If the answer to both was yes then go check it out, if not run like hell.

I know some go crazy for modded stuff and others wont touch anything thats not 100% stock, but if you know what your looking at and have interests in modding etc anyway you can get some seriously good deals on modded stuff, but if you are trusting the seller to determine the condition and value you can get into a lot of trouble from unknown problems.