Most of the time, the warranty is only to the "original purchaser".
My thoughts on buying a used regulator are simple. Unless you're getting it for $300 or more than what a new one costs, you're not getting a deal.
Here's why:
Typical regulator service costs is about $60 bucks for labor and $60 bucks for parts. So if you buy a used one then you'll pay $120/year on regulator service. You'll do it once before you ever dive with it and three more times over the next 4 years, which is a good average life of a regulator before people buy a new one or upgrade.
However, if you buy a new regulator, most come with "Free parts warranties" as long as you buy it from an authorized dealer and get it serviced annually. So this saves you $60 per year on parts. However, you still have to pay the labor, which usually runs around $60 bucks, but differs per shop.
So to me, to buy a used regulator, you pay $120 up front for the initial service and then an additional $60 per year on parts. If you take your first $120 plus the parts for an additional 3 years, you have spent an EXTRA $300 on that regulator over a new one. ($120 initial + (3yrs x $60parts) = $300 ). Now if you paid $250 for that used regulator for example, add the $300 and you've got $550 in it in original purchase plus extra service and parts.
for $550 to $600 you can buy a pretty decent BRAND NEW regulator that you know it's complete history on.