This is a tough one this day and age. Everything can be counterfeited so easily. I've spoken with some of the dealers I know personally and they say that anymore you might as well just take a personal check and wait to ship the gun until after it clears. At least on a personal check you have a persons phone number, address, and bank account information. The few cashier checks I've taken haven't had this information on them and when I asked my bank to check on the funds they said they had no way of doing so and I would have to deposit it and see if it clears.
Money orders are almost worthless I know of several people that have burned by phony Mo's for stuff they sold on EBay.

The Caveat about "buyer be ware" also applies to "seller be ware"