I just sold a pistol on GB and the ad clearly specified I accept POMO only. The buyer wanted to use Paypal and I told him I only accept POMO. He then sent me a Western Union money order, so I figured I had two choices: return the MO to the buyer, or cash it and wait for it to clear. Convenience store money orders are stolen all the time and I didn't want to wait another week for a POMO, so I deposited the MO and told the buyer he would have to wait a week for it to clear, per my bank.
Having said all this, it's easy to check a POMO by phone, or at any PO. Other money orders are quite often counterfeited or stolen and you won't know it for a week or more. However in the OP's case, I can't understand why he wouldn't take cash. Cash is always king, and the only option I can think of is that the seller might have been afraid the buyer would give him cash and then try to rob him. Unlikely but I can't think of anything else.