Going to the Post Office is a huge hassle. Even when I don't have to go to them, but say, FedEx Kinkos, I have to deal with them instead (I literally had a FedEx Kinkos employee punch the top of my box repeatedly to prove that it wasn't taped up well enough for their transportation, to which one can only think "It's not my bloody fault that you don't treat packages with any respect.")
I think a great deal of the anxiety in selling is not collecting money (eBay and Amazon both make this trivial) but the worry about holding up your end of the deal, and whether the lost opportunity cost of the hassle outweighs the value of doing the deal.
I know I just put everything in a giant box to Amazon's trade-in system because I only have to worry about shipping once, even though I'd probably get far more if I sold it all piecemeal.