are you wanting to leave negative feedback for the buyer that has left you a negative?  If your answer is YES, then you have the answer to another question:-  "why cant I leave negative feedback?".

Many years ago, when sellers could leave negatives for buyers, there were sellers that would leave retaliatory negatives for buyers who left them a negative.  Buyers became unwilling to leave negatives for fear of receiving a negative in reply.  This undermines the feedback left for sellers, which is used by so many buyers as a determining factor for whether to buy from a seller or not.  When buyers would not leave negatives it meant that bad sellers still looked like good sellers because they retained 100% feedback rating.  Unfortunately, the bad sellers from years ago who did leave retaliatory negatives have meant sellers can't even leave negative feedback for non-payers and genuinely bad buyers.  BUT, this is the system that is in place now, and by listing your items you are agreeing to abide by this system (not like it, just abide by it).

In your case, I would have refunded the PayPal payment and told them that PayPal offers NO BUYER PROTECTION because it was a pickup item.  I would have told them that their best buyer protection is to inspect the item before paying cash, and if not happy with the item upon inspection then walk away without paying (as all pickups should be).  As it is, you have asked the buyer to pay an additional $65 for the PayPal fee, which probably gave them the impression you were not happy with the final price and wanted more.

If you leave the buyer a +ve with a negative comment (a false +ve), you will receive a policy vioation if the buyer asks eBay to remove it.