Yeah, it's not like the car it supposedly came off of should have a 98' throttle body.
So I think it's your mistake for assuming (I probably would have done the same thing though), but he appearently "lied" in his auction, so screw him, get your money back however it takes.
Edited: typing while you posted
That sound good, glad you aren't gonna get totally screwed. It may have been an honest mistake on his part. His willing to give you some money back helps support that theory. Hopefully he actually does send you that money.