JMO, but base plates is one thing I would not want to buy used, anywhere.
They are sooo specific to a particular make, model, year. And, many of them require quite a bit of work to install, and just as much work to remove. Doesn't seem like a very good candidate IMO, for buying used. And, thats without even getting into any concerns about whether a used set may have had its strength compromised one way or another that you wouldn't be aware of.
Best place I've found to buy base plates, is
Hitch Source. Most can be had for around $400, and includes free shipping. Good place to get your other towing equipment (tow bar, supplemental brakes), too. I've had nothing but good luck working with them.
Craigs List would be another option, though, for getting tow bars or supplemtal brake systems used. I wouldn't try getting base plates that way, though.