Yes, the post office will confirm that the money order is valid and that you are the intended recipient (the person that the money order is made payable to) by checking your ID before giving you the cash. It's the post office's responsibility to ensure that it's valid before redeeming it. If they fail to do that and find out later that the money order is no good, then that's their loss, not mine, as far as I'm concerned.
IANAL, but it seems to me that they would have to sue me in civil court if they felt I was responsible for their dereliction of duty and the resulting financial loss. I'm pretty sure a counterfeit money order would also be a matter for the criminal courts. If they contacted me to inform me that the money order was a counterfeit, then I would be happy to assist them with their investigation and help them find the scammer, but I would not be willing to assume financial responsibility for their mistake.
Speaking truth here. I have cashed plenty of checks at the PO, never had a problem, never seen a counter fit while doing it this way. I used to work in retail banking and saw plenty of counter fix MOs, which is why I dont take to the bank. I also receive a fair amount of cash to my PO box. Always send with a tracking number!