I say this as a seller on eBay:
If they do that, they're asking for negative feedback and to have the money recouped. eBay almost always without fail sides with the buyer in a dispute. If the seller ships it after paying $25 for overnight and it doesn't arrive by the time stated, most buyers would open a case in the resolution center. And they would almost certainly get a refund through eBay, whether or not the seller is at fault is irrelevant. PayPal will zap the money from your account without your consent.