LOW, this is one of those often quoted but totally bogus comments often used to dissuade divers from buying used gear. Not only do I dive used gear but actually, I only trust used equipment. Here is the thing, equipment is new only once, on it's first dive and after that it is "used". Until I have had a piece of equipment on several dives, be it a new out of the box reg or a trusted reg that has just been serviced, it is not trusted until it has some bottom time on it. Now granted you know who used it last but really, what is the difference in a freshly serviced reg the you have been using and a freshly serviced reg from someone else. Nothing I can think of, for all practical purposes, new reg is simply a reg that has been serviced for the first time. I much prefer to know who the reg tech was than who dove it last. There is simply nothing to wear out in a reg other than the normally replaced parts during service. This is true of a high end modern reg or a 40 yo reg. Is it more likely that during service a reg from an unknown user it is found to have been abused, very possible but that is what a good service is for, the determine the internal condition of the equipment. My point here is that there is no valid safety argument to buying new vs used, assuming the used gear is serviced buy a good tech before diving.
That said, Ross, I do discourage new divers from buying off eBay and the like but not for safety reasons but rather for practical reasons. You can get some great deals but you can also get some very bad ones as well. To be successful on places like eBay you need to understand the gear, what models are easily serviced and what are not and what the service cost are going to be. Some regs are impossible to find parts for or can be worthless due to internal damage from a previous owner. There is no way to determine this via the web, you must have it in your hands to do so and it's sad to say but just because the listing says "great shape, just serviced and ready to dive" there is no guarantee that it actually is...which is why we all recommend having the reg at least checked by a good tech before diving it. A much better alternative is to check the local sales papers for equipment you can actually look at. If possible take someone experienced with dive equipment or better yet arrange for the seller to meet you at a dive shop of your choosing for the tech to give it the once over. You can get some great deals buying used and done correctly there are no safety reasons not to buy used. Just plan on spending another $75 to $175 servicing the gear.