A question about selling props

I was recently reading the un-ban request page, and saw that someone was banned for selling props, so apparently it is a rule.

So, I would ask, is it only against server rules to sell random props? I could understand not being able to sell things like turtles and fake guns but, what about things like beer? I know it's against the combine rules to, but would it be banable/kickable to start a black-beer market?