Ebay has done this to me, but only on a cheap little lens hood. They erased the auction with no notice to me or the seller. Fortunately it was delivered, the seller was legit (despite being in china) and all was fine.
So many fraud auctions are on ebay at any given time that they can't keep up-
so they have some heavy handed methods that often wipe out legit auctions.
I typed for half an hour the description and links on an item I was selling, and ebay cancelled my auction the next day, with no explanation, I still had to pay the fee..
Turns out I had a trigger word in my description that some automatic filter caught and automatically deleted the auction.
US postal money orders are relatively safe, because failure to send the camera after the seller gets the money order comprises mail fraud, and the post office will help you in that case. The post office will find you, if they really want to. As long as the recipient is in the US. They will do things like not deliver mail to the guy until he sorts it out for example.
Western union might have some fraud protection, I'm not sure.. don't see how..
Just because someone wants to be paid by something other than pay pal, does not automatically mean it's a fraud. Pay pal takes a bite out of what the seller makes, also leaves a paper trail for those who would rather not pay tax on item they sell. Not paying tax is fair I guess, if one paid sales tax on the new item when bought years ago, then why have to pay income tax on it years later, after having paid income tax on the money to buy it with in the first place?
If the M3 auction made it to the end- or had a buy-it-now, that caused the auction to legitimately end, where you got email saying you won the auction, and the seller charges shipping, then there is a fair chance it's not a scam.
Look for past auctions by that seller name, if he's been selling related camera gear and has good feedback, you may be ok, but if he's been selling nothing but collectible china, then an M3 out of the blue, oops!
The scams are the ones that are buy it now for some insane low price, but make you email first, they want western union, and have some too good to be true free shipping. Mainly they want you to complete the deal oustide of ebay. Also the scam auctions are in the wrong place, like a Leica camera in the car tires catagory.
Hopefully by now the seller has provided the tracking number.