It's easy. When the m/order is purchased they receive a receipt along with it. Have the buyer send you a picture of top part of money order along with receipt. Since the receipt shows amount purchased.

You now have m/order number, receipt to validate purchase amount, date of purchase etc. With that info when payment is in transit, call usps to verify, if you still need to feel comfortable about it.

For ticket items over $150-200 i take postal m/orders. IF they want to pay by personal check they already know (in written communications) there is a 10 day hold till it clears.