You hold down the right shoulder to stay in the inventory mode, then move the cursor over to the npc's barter tray and press A to select the items you want to trade. The little light next to the items should illuminate. You can also switch out items quickly on your side the same way.
I'm also having some stacking issues. It seems random if I'm able to add sling stones into a stack or not. Sometimes they merge fine, other times they will not. I have about 6 individual sling stones in my inventory and two stacks that won't accept each other or the individual ones.
It also happens with potions. Are they supposed to stack? I'm pretty sure I stacked two reds at one point, but none of my greens will stack