There was a vendor whose SR and forum accounts got hacked about a month ago. He created a new forum account and several of us said that he had to prove his identity, because the alt account could have been the hacker too. He signed a message and we waited for some of his customers who would already have his key to verify it. I don't know what the resolution of that was, since nobody came forward after several hours and I didn't keep track of the thread. The problem in that situation was that he wasn't a big time vendor and didn't have enough customers with his key to immediately verify his identity, but you can see that the solution is to distribute your key and trade keys with as many other people as possible. A top vendor would have been verified quickly and easily. Thousands of people have DPR's key, so it's basically impossible to impersonate him short of stealing his private key.