In order to sell to other players, you have to purchase a market board from one of the village merchants in your home town. Then you set yourself up in your room, price your wares and wait for people to come to you.
The stuff you make at level 5 is fairly simple for any player to make, you can try to sell it from your board, but do not be disappointed if you don't sell much. I have found the stuff that sells is the sub-combines to people who do not want to spend the time doing them (fossil temper, burlap yarn ect)
As to pricing your stuff, don't undercut the NPC merchants, that is a waste of your time. Price yourself over what an NPC offers you. You should also search the brokers occasionally to see what other people are getting for the same merchandise.
marten