How do you trade? (spoilers)

[GoreTiger]

I'm talking to the trader in the first settlement I encountered after getting off the beach. I'm trying to buy an armor piece. I have the armor piece selected, I put the exact amount of my gold that he wants for the armor in my offer, yet it's still refused. What am I doing wrong here?

[Gingermonkey]

Just double click the item so it appears in the center column , then click the scales to auto add you money into your side of the column, then click trade.

[Zerostalgia]

It defaults to "barter" mode which is a slower process that allows you to gift items to imrove disposition.
Just to the left of the middle of the screen, above your inventory there is a toggle from barter to trade.
That allows you to double click to sell items quicker.