Buying used can save you a lot of money. That said, here are some ideas about doing it:
1. You need to know something about what you are buying. There is a lot of dive gear offered used, and as Diver0001 has already observed, some of it is worn out or old enough that it can no longer be serviced. Your incredible buy on a set of Dacor regulators won't seem so good when you find out the company is out of business and the regulators can't be repaired. The tanks that were so cheap aren't a buy if you can't get them filled. The wetsuit somebody was selling for next to nothing won't be a buy if it's been through so many compression cycles that it doesn't insulate well any more. Shopping for used dive gear without any background data on brands, types, and possible issues can be a minefield.
2. You need to be able to evaluate fit properly. Some things HAVE to fit to work properly -- masks and wetsuits are among them. You need to be able to evaluate the fit of a mask correctly (and some shop employees can't do this properly!). You have to know that a properly fitted wetsuit probably won't be very comfortable to have on when you and it are dry, but if you try to buy comfortable on land, you will likely diminish the insulating capacity by not getting a close enough fit.
It's also true that buying things that have to fit either on line or via something like Craig's list, can involve a lot of frustration, because you will be dealing with one item at a time. Going to a large dive shop, for example, will allow you to try on a bunch of masks in a short time, and let you try several brands of wetsuit to see whose cut works for your body. This beats either driving to somebody's house to try stuff on, or buying on line and returning stuff over and over again, which can quickly chew up a lot of the savings you were hoping for.
3. Include service costs in your estimation of savings. Dive gear, in general, isn't horribly complicated. But it involves a number of moving parts and a plethora of Or-rings, and all of those things wear out. When you buy BCs or regulators used, unless you are a good gear tech yourself, you really should, for safety's sake, have those things serviced before you dive them. This can add a considerable amount to the cost -- our local shop will run about $125 for service on a full regulator set.
Some items are generally good buys used. Fins, for example, can be quite inexpensive and the things that would make them a bad purchase are generally easily seen, like cracking. Computers can be good buys used, again assuming you know what you are buying. And in contrast to some of the advice above, I think suits can be a very good buy used, if you know what fits you. Dry suits often go for peanuts used, and even if you have to pay to put in a whole set of seals, you're ahead of the game. Sometimes even if you have to replace the zipper, you're still okay! (I've seen lightly used DUI suits go for under $500.). Lights are often good buys used, particularly if someone is getting out of the sport, or there has been a technology change. I picked up several HID lights for ridiculous prices, when people started upgrading to LED.
I would even go so far as to say that people who are regular, experienced divers rarely buy anything brand new (except maybe dry suits). But there's a whole lot of background data and often a lot of acquired DIY skills that go into getting to that place. If you are going to buy used, try to get the assistance of someone who's been at this a good while, to make sure you don't buy stuff you will bitterly regret purchasing.
Of course, that can happen when buying brand new from the dive store, too . . . Review the "What did you buy that you regret?" thread. It's very long and not all of it is even on topic, but there's a LOT of good information there, if you look at the themes.
