Getting into diving can be a very expensive proposition. I myself didn't have a $5000 to throw around and figured that if I dug around on eBay long enough and did my research I could spot some solid equipment save some money. Well its turned out really well and figured I might as well share the experience.
First off BC's are fairly easy to find. I've swapped out on eBay several times with just about every brand. The two I like the most are the Zeagle Rangers and the Mares Syncro (Rec) or Dragon. I really like these regs and they are sturdy BC"s that you can get a great deal buying used on eBay. I have purchased several and here is what I have learned.
Two big items to look for on the Zeagle ranger is does it have a chest strap and what size are the shoulder buckles. Older one don't have the chest strap and have smaller buckles. It hard to swap shoulder sections with these. I would avoid them unless you just get a killer deal. I have one of these and it dives great but the chest strap is nice for an instructor wanting to pull you around where he needs you. Another thing is the Zeagles have a tendency for the glue holding the bladder to come undone. Get pictures of where the inflator hose connects to the BC and see if its come loose. Its fixable but I'd steer clear if you are a novice when it comes to glueing things.
The only real thing I've noticed that affects the Mares and Zeagle or any BC for that matter is look at the inflator. Get them to take it off and look for corrosion of the pull wire, valves, and hose connector. Plan when bidding to have to purchase a inflator hose and low pressure hose. As well as some cleaning.
Regulators are a piece of kit I was really scared to buy. I've learnt not to buy regs that are apart or not connected a first stage. Unless its new of course or getting sold separate. I've purchased many brands and the one that sticks out is Mares. In particular the proton series. I don't know why but I've purchased several of these and they all work, all the time. Can't say the same for any other brand. The last two sets I've purchased I got with everything including gages and computers. Absolutely no problems other than taking one of the octo's for service for some minor free flow. It was a $15 repair and is as good as new.
Dive weights and belts. I don't buy weight anymore. Its a pain and easier to just pick it up local. Belts can be had cheap. The only good deals I've got on weight was entire scuba lots.
Lights. I've picked up a number of really nice lights. The big thing to look for is assume you'll need new bulbs. I've never purchased a poorly treated light but I have gotten a light with bad bulb. They don't last forever. Also check the inside view of the light. I've never gotten a corroded light but I'm sure they are out there.
Wetsuits. I've never purchased a wetsuit or drysuit but booties, hoods, and gloves are plentiful and really easy to pick out using a pic or two.
Dive computer . There are two main types. Computers that have PSI and more of a dive watch that doesn't have PSI. My favorites are the Suunoto Cobra and Vyper. The cobra has PSI and the Vyper does not. Both are similar in function. Be prepared to spend some money on an interface cable for a computer if you want to connect it to a PC. Most do not come with it. Also be mindful of what it takes to replace the battery. Some are a kit some you can just buy the battery.