Postal Money orders are legitimate, they can also be called bank drafts.. Basically how they work is you pay upfront, and ship the money order (you keep a copy when you finish filling out the details on the money order, sending the other half to the seller). THe seller can take this to their bank and cash it...works much like a check in that sense but without the wait to have it cleared...since you can't get a money order without paying for the full value of the money order first, unlike a check). The cost of a money order itself, is usually pretty low...$2-4 bucks...been awhile since I bought one.
I have used them myself several times to pay for items I bid on at Ebay, finding it much easier than using personal checks. Email bank transfers are good too...
However, whether the balls this guy is selling are worth paying for...really depends on the ad and what information you can get from the seller. I would at least ask for current pictures...maybe requesting the snakes be on a scale or near a newspaper with the current date showing (this way you know its not just some picture they copied from somebody else, or 5 years old or something). Also you can get some idea on the snake's health, or at least spot any obvious signs of illness or poor care...like skiny, unshed skin, scars or open wounds (ask for belly shots too)
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PHLdyPayne