Another reason for vendors to have PGP keys is that it's their proof of identity in a pseudonymous community. There was a case where a vendor's market and forum accounts were hacked. He had to create a new forum account to inform his customers about it, and the mods requested that he sign a statement with his PGP key. There are a variety of situations where a vendor would want to prove his identity, and a PGP key is the best way do it.