The Barter skill affects the prices you get for buying and selling items. In general, the higher your Barter skill, the lower your prices on purchased items.

The higher your Barter, the more Caps you’ll get when you sell things, and the less Caps items will cost when you buy them. In addition to this useful bonus, Barter often acts as a Speech substitute… allowing you to haggle for better deals and rewards. However, Barter will not cover all your Speech checks (and vise versa), and Repair may actually do better at saving you money. In Hardcore mode you typically have to find things to sell to make Barter effective… which means exploring or hunting/killing enemies, which takes time, and hence, resources. It’s a fine skill if you have the points to spend, but it’s usually one that can be left for later.