The User is able to get a better price in shops when selling or purchasing. Think of it like free XP since you're selling stuff anyways. Bargaining doesn't fit into any Skill Category, so won't incsrease any stats.
When in most stores, “use_skill bargaining” will provide a little bit of information on what the store is interested in and whether or not you can expect reasonable prices: “This store trades in a bit of everything. It tends to overcharge. You think the owner likes you.”
When readied you will get a better price in shops when selling or purchasing. When the shift key is hit you will get some minor info about the shop you are in.