First of all make sure the models you are looking at are current in the manufacturers line up. Many of the older models are no longer supported by the factory and there are no replacement parts available.
Secondly make sure the arms extend and retract easily and there are no signs of damage. It's easy to bend one of the extension arms by backing up the motorhome with the toad attached.
If everything checks out make sure the price is only a small fraction of the new retail price. You will have no idea how many miles the thing has been used, and no idea of how it was treated regardless of what the previous owner tells you.
Keep in mind a damaged tow bar or one that can't be repaired due to lack of parts is nothing more than an expensive paper weight.