Used regulators are another thing. My thoughts on this are simple. If you buy a new one you typically get "free parts warranty" when serviced every year. If you buy it used you get no free parts warranty. Typical labor on service will run $50-$60 regardless. If you don't have have free parts warranty, then parts could typically run $40-$60. So getting your "used" regulator serviced could easily be $100-$120.
So if you buy a "used" regulator and get it serviced when you first get it ($120), then ever year for the next three years, you're spending about $480 bucks on regulator service (or about $240 on parts alone). so did you save $240 bucks by buying that regulator used? most likely not. maybe you did though. I use that as a 'break even point' to figure out if it's worth buying it used. If I'm only saving for example $50 bucks by buying it used, over the next three years I'll be paying an extra $240 for service and in the long run spending more money. So I'd buy it new unless you get a steal of a deal on it. Also buying it new you know exactly how it's been treated and you don't typically used."