It is easy to be cynical. However, in my experience most people are trustworthy. I must have bought several hundred items on ebay and only a handful have been problematic. And usually the scammers are pretty obvious - eg the repeated ads for a pre-war Model 18 Norton starting at 99p. But I always report these and they disappear promptly.
I must say that in my view a few clicks to buy bike parts is infinitely superior to wasting half a day standing in the queue at Vale Onslows on a Saturday morning only to be first insulted then told the part you want isn't available.......