I went and looked at the ad and this is what I figured out.
1) That pic looks like it has been taken out of a magazine or something. It is too clean for a non professional photographer.
2) I would ask him exactly where that engine is and if you can have the name of the company or person that has it in their possession. If he gives the name to you then call them directly to see if they have it.
3) no phone number listed on ad. Red flag there.
4) Going thru autotrader for the deal sounds like a scam to me.
5) New member. Joined less than 2 weeks ago.