The problem is not implementation, that would be extremely easy to do, it's the Paypal fees. Paypal would be taken anywhere from 5 - 10% of each transaction, so if you're spending $100, they're only getting $90, add on top of that the problems with paypal charge backs and fraud, it just isn't worth it for Newgrounds who already make little from purchases.
Personally I'd love to be able to buy with Paypal in the NG Store but they don't have it because of business, not they just can't be arsed to implement it, although maybe if there was enough interest they'd try it out :)