Not so much problems for buyers, just no buyer protection should it be not as described.

They come to see if first then pay. Gives you more credibility to say you prefer they see it before they pick up. So no benefit.

It's more a potential problem for a seller if the PayPal payment was funded with a credit card and they do a chargeback claiming fraudulent use months later. 

Most won't insist when they learn there is no benefit, but if you get one that does maybe because they want to use a PayPal balance, can't flatly refuse it.