You probably can't. You could try going to the place you wired it from and asking them not to send it, but for the most part, once it's wired, it's gone.
It is a prepaid check that guarantees payment from one individual or group to another. It isn't necessary to have a checking account in order to purchase and send a money order. Whereas checks can...
It is totally up to each eBay seller to state what types of payments are accepted. The seller can also decide for each individual listing (if he so chooses) what payment methods are accepted.