Items in a trader's inventory can also be altered (not referring to the unlocking the ability to be able to trade) by quest or dialogue as well. This works both ways in a few rare cases. And KyaSapphire is correct. It is easier to spot when in dialogue mode with an NPC. There are by default a set of buttons to the lower left (on PC anyway). Those that you can trade with will have one that allows you to open a trade window. There is also a button to switch to a party member that is not in the dialogue. Useful if the conversation is not going in your favor, and your companions may need to prepare for the inevitable. Or perhaps one of them wants to explore what is behind curtain number 1, while the NPC is engaged. Maybe similar to what one parent was doing, while the other was setting expectations meeting your first date.
