The Herb shop is run by the Shop keeper. It sells all supplies for training farming.
The herbs are in order. If you are starting farming new get a couple of the first herb. Get all herbs and use it on the patch to see if you can use the next one in order, if you can it will tell you on what level you can. Also in the farming skill there is a little difference to it. Your level goes up to 99 like all other skills however your xp stays 0, so dont get scared if you recognize this.
