Outstanding reply Bubbletrubble.
You can sort of think of a used regulator like a used car: If the used car was properly maintained and was a "good" car to begin with, and the price is good, then it can be a great deal.
You do need to know what your are looking for (brands & models) and know what you are lookind at when you examine the reg (excess wear and tear, corrosion, is this the actual model the seller says it is, etc.).
With used regs you also need to give some thought to where you will have them serviced, and of upmost importance, can you still get parts fot it?
It is a real bummer to buy a shiney, well-maintained reg, only to find out that parts are hard to come by.
The last 3 regulators I bought were used, and I bought them on ebay. They were amazing deals ($135 for a regulator that sold for $690 new, $65 for a regulator that sold for $450 new, $150 for a regulator that sold for $690 new).
I knew, before bidding, what these regulators sold for new, that they were very good regulators, and that I could obtain parts for them easily and cheaply (I do my own servicing).
As Bubbletrubble mentioned, have a friend who knows about regulators help you if you decide to buy used.
Best wishes.