If it's for a lot of money, send registered.
If it's not and you couldnt be arsed chasing a refund in the event they run with ya money, I'd send it normal Air Mail.
Western Union MO's can be traced, I've had to do it once, but after I sent the Seller an email saying I had the # and was in the process of contacting WU, the card arrived within a few days.... (after about 30).
Good luck with it.
Last time I accidently did that (had a sniper set after only looking at the auction once, and didnt look for PayPal acceptance), I contacted them and said I couldnt pay for it, would they mind re-listing, or offering 2nd chance. I said I'd PayPal any small charges they got on the "sale" from me. They were cool with it and just re-listed it.