I bought mine on ebay. The biggest things to me (other than price and condition) were the number of SALES by the seller and the reputation. In my case, it was a dealer who had sold a mix of exotic Euro cars, muscle cars, and Ducati and Triumph motorcycles, with a 100% rating on 90+ sales. More so, I emailed him with a phone number and found out what his bottom price was (below his "buy it now" price). I accepted, he told me to bid, and then closed the auction early. All in all, a mostly successful transaction.
Downsides: I didn't get to see the bike first. There were things that were described, but it still would have been better to see first. That the bike was very new and very low mileage was a big consideration.
In general, I think vehicles on ebay go for wayyyy too much. Better deals are to be had on Craigs List and I have done far more there (buying and selling) than I have on ebay (at least for vehicles).