A coworker of mine has purchased at least three or four motorcycles on eBay. The only logical thing to do (what he did in each case) is to pay a deposit when you win an auction and the remainder when you pick up the bike after inspecting it to make sure it's as promised.
Personally, I would never pay for a bike before seeing it. I wouldn't buy it if the vendor wouldn't agree to that. Don't pay for a bike you've never seen, just like you wouldn't buy a bike on Craigslist by sending the vendor money through Paypal.
There are so many motorcycles accessible at dirtbikeadvisor.com and locally that I don't see the need in purchasing one from afar.