You can prevent getting friend / trade invitations by setting your state to do not disturb (/dnd).
The notifications seems like a good idea, maybe just a changed marker until you select the item. At the very least, every time the other player makes a change your accept button will gray out for a second to prevent you from accidentally accepting a trade. Adding another 3D preview window into the UI is a bit tricky, especially in the location you're suggesting, so likely not in the first revision of the trade system. You can definitely open the preview window by using the button in the item details.
The current design does not have a trading fee. Though the items do need to be unused or refurbished to be traded.
You can trade all the stuff you can normally trade. We will be allowing you to trade legendaries through this system as well.
Sounds reasonable, I'll look into what we can do here.