when browsing a venders items mouse over a store item u wish to get the value for so that it highlights in white then move the mouse over to your side of the divider over one of your categories so it highlights white also (so store item and one of your sell categories are highlighted) then careful mover the mouse to the 3d model of the item without going over any text so they stay highlighted then left click u should see your money go up and the vendors go down, because u are selling there items back to them.
Also if ya clever u can use this to dup items.
p.s. YAY I was first and not even a full day after its release
To developers I think it has something to do with when a category is highlighted depending on which side of the divider its on determines if its selling or buying the current item
I find this a cool glitch kinda a secret cheat so please leave it in for people thanks

