bait and switch car parts

Oh, yeah I see what you're talking about. They're clearly pulling one over on people. If nothing else, you learned a lesson. You could certainly try filing a claim with PayPal, or even try a charge back on your credit card. You just have to decide if it's worth the effort.
I learned a lesson. The seller said he'll do a return, and I'll take him up on that.
He did try to lecture me about gaskets but I wouldn't hear it.