I just posted some of this on another thread, but I don't mind repeating.
Buy used. My first reg set was bought used from my LDS. A good customer was moving out of town and they took it (uncharacteristically) on consignment. Over ten years later it's still my favorite reg.
Craigslist, and garage sales are great places, but you have to know what you are looking for and how much to pay. I have bought a 4 or 5 reg sets from Craigslist and a number from garage sales. Had them serviced and they are regs for my son, pony bottles, and backups. Just bought two pairs of Apollo Split Fins for $50 a set. Bought original BCDs new from LDS, but since then have bought used (like new) BCD from Craigslist. Also have bought 3 steel HP 3442 tanks from Craigslist, and 3 new from my LDS. Also bought 4 Al 80s from Craigslist, and 2 new from my LDS. I bought my original computer (which I still use as my primary) and a used one/backup from Craigslist.
You can get great deal on Craiglist buying from a old diver or someone who just had this stuff laying around - but alot of it is just junk that will cost you more to service it than it is worth. I used to buy from Ebay, but today I'd advise against Ebay, as it is full of charlatans who are buying stuff really cheap and then splitting it up and ripping off people. Now, "1 in the million" you might get a good deal, but mostly you're just wasting time.
Now that I've said the above, I'll tell you these two stories. One of my friends has a brother who decided to quit instructing. I went over to his house (I wished I'd gotten there 1 month earlier for his main reg set) and bought everything he had left - really nice stuff - everything from a float tube to all kinds of hardware and a really nice big heavy duty box. Another time, I saw in our local newspaper an ad and I decided to drive by the day before to be able to find the place since it was an odd address (I hate and do not do "early turd birding"). I saw an older couple struggling to put together raft and could see that they had mis-assembled something which would have made it impossible to put the rest of it together, so I got out and helped them. They were the nicest couple who had been diving their entire 40+ marriage. Afterwords, they invited me in the house and not only told me to buy whatever I want, they also showed me pictures of their dives around the World. I bought a number of Conshelfs that I dive with today (primary and backups), diveknives (including a pink one for my granddaughters - 5 months & 2 1/2 - I have to wait a while) - and not only that, we spent hours looking at underwater photos from around the World (no, I did not buy their Nikonos)!!!