Check his feedback, check what else he's been selling, is this seat a strange thing for him to be selling?
If you're buying off ebay, you'll get their buyers' protection too.
If you're happy that he seems to be a genuine seller, then send the cheque by signed for delivery. If you don't get your seat, then raise a complaint with ebay.
They'll have to prove that the seat's been delivered, so they'll need proof of delivery (Proof of postage isn't good enough) If that's no forthcoming, then they'll find in your favour & you'll get your money back.
If anything, eBay's biased towards buyers.
I had a buyer try & scam me last year. I was selling a gold watch, bought & paid for via paypal. Sent it through Royal Mail Special Delivery. The day after I'd posted it, got an email from ebay, that the sale should be cancelled, the buyer contacting me via eBay's messaging system to ask what's wrong? Nothing I say, watch's been posted. Then I get paypal freezing the funds, the buyer's closed their account, I get a non-delivery complaint raised. So, responded to that with the Royal mail's proof of delivery, to the buyer's registered address. Ebay OKs it all, money released.
The only thing I wouldn't use Paypal for, is collection only items.