You need to give traders gold / items for free to up the attitude. Higher gives you discounts and is worth having at least one trader in the green from what I've seen. My go to is to simply slowly give them gold until in the green and then toggle to the Trade mode (off Barter) and click the sell all wares button. Then toggle back into Barter mode and buy things.
I've been keeping some greens for utility etc. The real problem I have is books, that now get transferred into another backpack in camp storage. Don't want to sell them as some contain clues etc. but they do have weight. Not knowing which items can be used for other things in general. Do I need to keep a hammer and tongs in my bag, maybe an apple on the off chance someone asks for one.