If the merchant is at home with the wilds, then it is all right to barter with him. If the mechant is not at home with the wilds, then you shouldn't barter with him. You can see what mobs are in home with the wilds by considering them once you are a member of the cabal. You are also able to request from mobs who are at home with the wilds.

Personally, I think Outlander bartering with a mob that is not at home with the wilds is slightly worse RP-wise than Maran paladin buying stuff from an evil guy.

Outlanders don't have to follow any rules. You joined the cabal and got in just because you happen to agree with the stuff that's written in the bark. You don't just obey it because it's written in the bark. Personally, I think that Outlander has no place for people who think like "I must act that way because the bark says so." You must understand why it says so and agree with it by your own volition. If you don't agree with some rule in the bark by your (character's) own volition, you don't live by it. Outlander also isn't a place for bark lawyers who tell other Outlanders to live by the bark appealing to the bark like it was a law. Anyway, you can do whatever is ok according to your role, but I think that an Outlander that barters with city mobs not in home with the wilds is a bleh Outlander.