30 bucks for an SE/30 is a deal, it's the pinnacle of the B&W compact Macs. Unless it's just ragged 30 bucks without having to pay for shipping is a good deal. Go get that thing man!
If you want the SE/30 then go and get it. $30 = £15 in my money which I would happily pay for a good SE/30 that was nearby, (if I didn't already have one). Try a wee bit of haggling for old time's sake, offer say $25 and see if he'll accept.
Are you kidding? $30 bucks is a steal. It would be an insult to offer less. Just grab it and run. If you don't pay him the $30, he can easily find someone else who will.