You will get all kinds of subjective opinions on this : some will tell you FedEx, some will tell you to avoid it at all costs, etc. The important thing, imho, is to pack it up as securely as you can; don't scrimp on packing material.
i buy and sell junk on ebay, (1550+ feedbacks) ups ground seems to be cheapest for larger items like speakers and heavy amps. i havent checked greyhound, but i think the receiver has to go to the bus station. ask the buyer if that is ok.