Vending is fairly simple. You can vend in any realm, but it is perferred to do it in the Trading Realm where everyone gets together and sets up shop. To set up a shop open your bag and click 'vend' . You can buy as well as sell in a shop. To buy, you have to set up what you are buying at what quantities, and for how much you are buying it for. People can then come and sell their item to you for that exact price. For Example, a Mystics sets up a shop buying ethrelics for 8s and selling them for 15s. A Shaman has an etherelic he is willing to sell for the 8s, and he can click and drag his ethrelic into the mystic's store and lose his ethrelic and gain 8s. The selling mystic loses the 8s and lets say he was buying 80 of them. The shaman sold one, so the count now goes down to 79. A Dragoon comes up to buy an ethrelic, and sees the 15s and deems it was a good price so he clicks and drags, types the quantity and buys the etherelic for 15s.
You may not set up a shop and roam around at the same time. If you set up a shop, it's similar to games like Flyff where you have to just stay there while vending, and when you are done, you can close shop and move around agin, or if you just feel like finishing, you can close shop too.