I'm sure there are lots of opinions on this, but for me, I wouldn't take the chance buying a used towbar. The manufacturer of mine recommends "service" every year or two if I remember correctly. If a new one cost $1k and a used one was $500, I personally couldn't justify saving $500 for something that has a lot riding behind it! For others, it's an acceptable risk.
Don't get me wrong, I buy used stuff all the time, but in my mind, some used things just aren't worth the risk. Fire Extinguishers are another thing that comes to mind!