I've had someone ask me once to ship them the item first and wait for him to receive it before he'll pay me. I thought that was a new one. But I agreed.
Worked out fine in the end.
I estimate that 20% of the time, people back out. I expect it so it doesn't really bother me. I also expect people to give me offers or ask me to meet somewhere or what not. Also doesn't really bother me. I can choose to accept the offer or not. I can choose to respond or not to offers. Generally, if I say price is firm and someone gives me a low offer, I probably won't respond but that's just because I expect the sender already knows that I am not interested. If someone gives me a reasonable offer, I may accept it or reply and say that I'll think about it.