@(you need an account to see links)
All sort of payments are accepted, depending on the person you are trading with.
I have once traded my UC + USD, for a better UC.
I don't think you should just ''give'' up on your UC for the trade to seem more legit, but that's just my opinion.
Maybe state in a selling post you already have an UC and you are interested in adding an USD amount to it in order to get another UC.
I guess you just need to find the person who's willing to do it, but yes, it's possible.