Bargainer

Is 'bargainer' a word? If not, which word expresses this meaning. I can find it on some online dictionaries, but not in other dictionaries. For example, he is very good at bargaining and he is a very good bargainer. Thank you!

It does show as a noun in our dictionary, so you can take your pick, can't you!
Our dictionary doesn't offer a definition, but obviously it means one who bargains.

Personally, I would probably not use it, favouring "good (or bad) at bargaining." if I felt the need to talk about someone who bargains.

I just checked the authority of the OED and they have examples of it in use dating from the 16th century, so it is not new!