i have been reading on ebay about western union money orders. i can call western union and verify that the money order is legit before depositing. if they say it is legit then i am probably ok. but i also emailed the buyer to say that i will not ship until i have verified that the money order is ok. if i don't hear from him i will have to go through ebay to cancel the sale and relist the computer. kind of a pain, but i am not going to get scammed. thanks for the advice - i feel kind of stupid for accepting money orders now, but i learned my lesson.
I would give the buyer the benefit of the doubt (assume everything is legitimate) unless a problem arises with the payment. I have received a money order as payment for an eBay sale before, however the buyer simply purchased a postal money order and mailed it to me, and I received it a few days later. For some reason, the idea of using Western Union raises an eyebrow with me because of things I've read in the past.
Of course, it COULD be a legitimate buyer and therefore I would assume so, unless a problem arises. Of course, do not send out the item (no matter what the buyer says) until you've been fully paid. And do have your antennae up since Western Union is a preferred method of scammers.
Edit: To be honest, I have no idea how Western Union actually works. From your description, it sounds like they act as an escrow. Personally, I'd prefer to have the money in my hands before I sent out a computer. Could you tell the buyer that you'd prefer him to send you a postal money order, and that when you receive it you will ship him the item? If it were me, that's the only way I would feel comfortable.