I use gumtree to buy and sell from time to time, buying is usually an easy deal, selling is bit hit and miss...
If someone wants to collect it i meet them at the local Maccas, never at home, for anything, ever. there are so many weirdos out there!
I always get them to do a bank transfer rather than PayPal if i'm posting it, and i always get their details before i give them my account number, I also always send them a pic of the tracking number and the addressed parcel.
The low ball offers and scammers are part of the entertainment, if they want to be rude i string them along, sometimes sending them to fictitious addresses to collect a "bargain" fuckers deserve it..... never hear them complain though, funny that!