Is there an inventory sort button? If there is, I can't spot it that easily.
Yes, it's a little bar with an arrow to the side of it. Click on the arrow and you can sort by value, weight, type, or time of acquisition.
I personally think it would be nice to be able to select multiple items in the inventory in a simpler and quicker way for trading with party members or bartering.
For bartering, maybe it could be nice to be able to pre-emptively mark items for trade so you can quickly give them to a trader. Saves time for multiplayer as well.
You can. Right-click and select "mark as wares"