Farming-
Okay, we have the farming shop and you have nothing to do with it.. so my suggestion would be to make a farming patch and you use the seeds that you buy on the patch and doing this gives you xp and a herb back to be able to use for herblore
Herblore-
I think you should have herbs from farming (no shop to buy herbs from) and use those to make potions so you have vials of water (can buy from a skilling shop) and you use the herbs from farming to make a potion.. different herbs make different potions.. EXAMPLE: Rannar + Vial of water = Attack potion Snapdragon + Vial of water = Strength potion and so on...
I belive that having these two skills tied into together will make the user have to work for the materials and not just buy them