Set and unique items do not show up in the trade window. On act change, they will be renamed (the stats remain the same) and then be included in the trade window.
Equipped items on non-lead character(s) are not displayed in the trade window. If you want to compare items for sale with what you already have, you will need to exit the conversation, select the character with the items to compare, then trade again.
Some items are not sellable, such as quest items, non-plant alchemy material and some gems.
