You are at someone else's mercy when buying used online. Until you get your hands on it, you don't know if it's as great as described. What I would look for would be about 50% of current NEW value. It should have all the features you want from a BC.
Once you get it in your hands, you need to make sure the stitching is solid, it has all it's components with it. Mount it on a tank and make sure it will hold the weight and that the power inflator and OPVs work. Make sure the manual inflator works. Inflate it all the way and let it sit over night. If it has lost a noticeable amount of air it has holes and is bad. If it stays inflated, then you have a good BC. This is a good time to shake it aaround and listen for salt crytals bouncing around inside the bladder. I have seen BIG salt crystals inside of some. These are dangerous and can cut the bladder.
Lastly, remove all the dumps, wash everything, rinse the inside well then hang it to dry. After drying re-install the OPVs.
This is my list for buying used BCs.