You two are kinda hitting on my unspoken motivation to this post and I appreciate the competing viewpoints. Because it is so obviously my lifeline while underwater, I don't want to cut corners and buy a used reg that is potentially dangerous. At the same time, I don't want to blindly interpret price as a proxy for quality.
Are rental regs GENERALLY worse than most of the regs on the market? It seems like on every dive trip I (or someone else) get one that free flows. I imagine that even if I get a less-expensive reg, but maintain it meticulously, it would perform better than a rental. Thoughts?
The first thing you need to realize is that your life does not depend on your regulator. If you dive in such a manner as to die in the event of a reg failure, you probably should not have been certified to dive. Safe diving ALWAYS includes a back up air supply, whether it's a buddy, or in the case of advanced (solo and/or technical) diving, redundant equipment.
Since dive safety depends on your judgement and behavior, NOT your regulator, used regs are not in any way more dangerous than new ones. However, used regs might need to be thoroughly inspected and maybe serviced before you should dive with them. Actually the same is true for new regs, although they certainly should not need rebuilding, maybe only adjustment for best performance. It's just that buying a new reg from a dealer typically includes that inspection and adjustment. Not always......but the point is that buying a used reg does mean getting it inspected and maybe serviced right when you buy it. But that's for convenience; you don't want to lose a dive or cut one short because your reg fails, and you want to enjoy diving without worrying about your gear. These are reasonable considerations, but it's not about safety, unless you dive in a risky manner.
Whatever you buy, you will need to get it serviced eventually. Serviceability is really important, and it's not always easy to evaluate a service technician. It's not difficult to service regulators, but it does take some care and attention to detail.
Rental regs are usually lower-end models like sherwoods, sometimes MK2s, often unbalanced piston regs with unbalanced downstream 2nd stages. These are typically very reliable and can withstand a lot of abuse, which they often get in a rental fleet. They can easily go out of adjustment and free flow, so can any reg, but rentals just lead a tougher life than privately owned regs.
But, for example, if regulators were really responsible for a diver's life, do you think anyone would dare to rent them out? Any idea what the liability would be on this? They free-flow, or a hose bursts, or maybe the mouthpiece falls off. What happens? The renter simply shares air, switches to an octo, and surfaces, probably pretty annoyed, but definitely not dead.