I've bought three used drysuits and sold two. I have one that I use as my regular (and only) exposure suit.
If I had known then, I would have just bought a new drysuit.
Nevertheless, here's what to look for:
0. The fit:
If it doesn't fit, it's junk. The End.
You should be able to put it on comfortably while wearing some clothes. I wear thermal socks, spandex pants, a dryfit shirt, and a fleece vest under my neoprene suit. If it's a laminate suit, you'll have to be able to put on some warm clothes underneath. Since cost is important, I'll let you know that under a laminate suit, I wore a $10 pair of fleece pants (I added stirrups) and a $10 fleece pullover. They worked great in 4C water. I've heard that despite the high price, the official $300 thermal drysuit undergarments are better.
The neck seal should fit tight, but not too tight. You should be able to get a few fingers in there, but it's got to form a watertight-under-pressure seal. You **MUST** be able to wear the suit for hours at a time without feeling uncomfortable. You will know within a few minutes whether the seal is too tight.
The wrist seals should fit about the same. If your hands get cold, then they are too small.
Overall, you should have a very high degree of flexibility. A drysuit is supposed to restrict the water's movement, not yours. Stand stall, reach your opposite shoulders, squat, jog in place, etc. Any tightness will make you think "freakin' suit" while you're underwater.
Watch the fit on the shoes. You will probably want to put on some nice, thick, thermal socks on before you try on the suit. The boots should fit like any normal (i.e. land) boots. Note that you may have to buy new fins to fit your drysuit boots.
Problems with the fit are serious. They will not get better underwater, and DO NOT BUY a suit thinking that you're going to lose 5-10 pounds. Buy for your current weight.
1. The zipper(s):
These should open and close freely. If they are sticky, walk away. That means that the previous owner doesn't use wax or doesn't own any, which means that the suit isn't being taken care of. My suit is a rear zipper, but the zipper's in such good shape that I can open and close it by myself.
Look for visible corrosion, too. Some discoloration is normal, but not lots.
2. The seals:
As I said during fit, make sure that the seals are fitting correctly. If they aren't, you'll get wet.
Latex seals should be even, with no signs of rubber corrosion. Ask if they will throw in the Seal Saver. If they say, "yes" or "no", then walk away. The answer you're looking for is "the what?" or "I don't use that." The silicone will be all over the suit, making repairs (and there will be some) tough, bordering on impossible.
It's harder to tell with neoprene seals. If they are flexible and pliable, then that's all you can hope for. If they seem brittle, the suit is really old and hasn't been maintained.
3. The valves:
There are two valves, inflate and exhaust. These tend not to leak internally, but leak around where they attach to the suit. If you're feeling ambitious, you could take them apart and inspect them, but it's unlikely that the seller will go for that. The best you can do is ask if they leak and look the guy in the eye when he answers.
Make sure they work before you trust your life to the suit. You must be able to inflate for a drysuit to insulate you, and you must be able to exhaust to control your buoyancy. You can test this yourself with what's left in your tank after a dive.
Most valves leak a little, especially the exhaust.
You may have to buy a whip; if it's included, ask why.
4. The rest of the suit:
If there are holes in the suit (e.g. loss of buoyancy above a sea urchin colony) they are easily patched. You'll have to check your suit once a year anyway to make sure there's no wear and tear.
Check also how it's stored. Is it hanging in the garage with a good hanger or rolled into a careful bundle in a storage container? Is it in a pile in the closet?
Remember that the suit is being sold because there's a problem with it. Find out exactly what that problem is before you hand over any cash. Most of the time, there's a good reason: my wife said to sell it, I got a new suit, I haven't done a winter dive since 2002 (warning!), I don't dive anymore, etc. Most of the time, the guy will tell you point blank what the problems are. For my suit, I knew that it leaked a bit when I got it, and I patched up the leaks.
Finally, there's no such thing as the deal of a lifetime. If it's a good deal, then another one will present itself. If it sounds too good to be true, it probably is.
