Maybe so, but it's a hassle I'd rather do without.
You seem to ignore the fact that as unenforceable as it might ultimately be, it'd still potentially involve lots of bank charges, letters from solicitors etc etc, in the mean time. Would you happily accept payment, knowing that something like that might happen, and if it was to, that Paypal specifically don't cover you under their seller protection policy for the type of goods that you've sold, meaning that it's inevitable they'll chase you for the negative balance on your account a lot sooner.
Some people's ignorant attitudes never change....