agreeing to and coming to an understanding of a fair price/exchange of one item for another.
Bartering can and does happen with money. An item is for sale at $10 someone offers $7, that is batering. Usually an offer is met with a counter offer, it goes back and forth till both parties agree on terms.
Most of what people are saying on here is trading to me, I will give you this for that. That is a trade. A barter is finding common ground on a price.