I have purchased at least 15 used regulators on eBay. I have NEVER had an issue with them. I always service them after I buy them, irrespective of any seller claims that they have 'just been serviced', or anything like that. I still have all of them, and use them. But, I tend to buy specific items - specific brands, specific first stages, and specific second stages, with which i am familiar.
Now, that is NOT a guarantee that you absolutely will not encounter a problem if you decide to buy used, it is only my personal experience, offered for what it is worth.
I do have the benefit of being able to service them myself, so the cost is somewhat more manageable. But, if you get a good price on a used reg, even after service it is probably a financial gain. One suggestion, if you decide to purchase on eBay: find a reg that you want, not just a reg that is for sale. Before bidding, get a good / valid 'new' price estimate (you can do this online). Decide what you are willing to pay (personally, I never pay more than 50% of a new, discounted - e.g. online vendor - price, and usually much less), and STICK TO YOUR MAXIUMUM PRICE - don't let the 'thrill' of the bidding process cause you to pay more, just because you want to 'win'. I have found many overlooked bargains, and I have also passed on quite a few regulators on eBay where the bidding passed what I considered to be reasonable.[