I have personally tossed allot of money away by buying used. You need to know what you are buying, or you will lose.
First and foremost, stay away from anyone that says they're not diving anymore, and clearing out their garage. Odds are good that the equipment is over 20 years old. Instead, go to your LDS and figure out exactly what you want. Then find someone selling that specifically.
I would avoid a jacket BCD. If you get serious about this sport, you'll likely need a BP/W. No point in tossing good money at a jacket BCD if you're just going to replace it in 6 months.
For tanks, you should be looking at steel. They're expensive for a reason. If you are looking to buy now, then spend more later, find someone with LP 72's. I got mine for $50 each, and they were already hydro and vis'd. Eventually you'll want HP steels, but that'll cost you a pretty penny. Even used, they're very expensive. If the Hydro and Vis is not current, get the seller to agree to give you your money back if they fail. Find a place that can do the Hydro/Vis for a good price. If you live in SoCal, I highly recommend Pacific Wilderness.
Get a good quality regulator. Since you're looking at used, you can probably get one that would normally cost $700+ for less than half that.
Go on Craigs list and look in the area where rich people live. Ideally you'll find someone who bought all new gear, and then decided they werent that interested in scuba. For me, that means looking in Orange County.
Best of luck.