The following page of our game guide to Wasteland 3 provides information on trading and bartering. You can find out, among other things, information about the whereabouts of the traders and vendors.
In comparison to the previous part of the series, the trading mechanics have been slightly simplified and they no longer resemble the trade mechanic used in the Fallout franchise. The whole thing no longer consists in exchanging goods. From now you have to pay in cash if you want to trade.
You can encounter merchants and vendors in any major human-inhabited resort, such as Colorado Springs, the Bizarre or Denver. You can recognize them by the dollar icon that floats above their heads. In addition, while exploring Colorado, you may come across random vendors.
Once you decide to start a transaction, you will be transferred to a special panel. The trade panel contains your inventory and all items offered by a particular vendor. You have to keep in mind that not every dealer offers the same goods. For example, if you want to buy upgrades to Kodiak, you have to trade with the mechanics residing at Ranger HQ.
After you choose what you want to sell / buy, then simply confirm your choice. The dealer doesn't have a limited budget, so you can easily sell most of the useless items you carry - mainly old armors, weapons and other sort of rubbish.
In return, you can use the earned money (your budget is unfortunately limited) to buy new equipment, ammunition or medical supplies. The price of goods is not constant. If the seller belongs to a friendly faction or your character has developed the Barter skill then you can count on appropriate discounts.