Although a money order is an allowable method of payment, the reason eBay doesn't let sellers demand them is that they offer no Buyer Protection against Item Not Received or Not As Described problems.
The usual Protection is a refund for the problem item. This is not possible with a money order.
Cash on pickup is fine, since the buyer has the chance to inspect before handing over the money.
Mostly left for other newer buyers who may not be aware of the potential problems.