Well, for something like that I would probably shoot for new, unless you were getting it used from somebody you know/trust etc.

I bought a Tekonsha Prodigy from a guy on ebay who sold them new. He had sold a bunch of them in the past and had 100% positive feedback. I got the controller with the F250 specific cable for about $110 shipped. Turned out to be a good buy, luckily. I probably would avoid ones on ebay that are advertised as "used" or sold "as is".

I don't know which unit you're looking at but the Prodigy is pretty popular. Mine has worked like a champ for me so far.

Good luck!
Jason