Basically what Lawdog said. I have only added (for the most part) retailers that I feel are responsible enough to keep age in mind when selling to customers. I don't expect them all to do it, but I'd hope that they'd make an effort. If they really don't, they'll be removed.
We restricted the buy and sell because 1000's of people use it. It's hard to ask every one of them to be responsible about selling their gun - the age verification system helps (and hinders, I know). At least retailers only account for around a dozen store owners, who have a vested interest in maintaining a good public profile.