There is definitely some mutual trust required of any of these types of deals, and that's ok.

In your case, it sounds more like tire kickers who got too far in to just back out with tact, so they pushed harder to get you to cut the deal instead. I see it a lot actually. I just recently offered to sell my older Xbox 360 to a guy on Facebook, he had this whole sob story about how it was for his little brother who wasn't getting anything for christmas, so I offered him a really good deal on it, including everything I had. He seemed perfectly fine with the price, but they wanted to only give me half up front when he got it, and half the following week when he got paid. I don't know if he just really couldn't afford it or if he was trying to rip me off for half price, but as a seller, you never have to play those kinds of games.

Set a firm pricing policy up front, make sure you follow it, require them to follow it, and keep your paper trail in order. If you have a signature showing it was picked up and it is the buyers signature, they're not going to get their money back from paypal or a credit card company. If the signature does not match the buyer, then FedEx made a big booboo, and that's what shipping insurance is for.