Guillaume Dery
Used saw vises are hit or miss and unfortunately often more miss than hit.

This is not something that I would buy online but in person where you can inspect it. I bought about 5 before finding one that I am moderately happy with (a Disston #1. Although I am actually still planning on springing for the Gramercy one as it looks awesome and I only hear nice things about it).

Things to look for:

-parallel jaws (should go without saying)
-clamping action (smooth, sufficient pressure)
-Length (longer is better)
-wear and tear on the jaws (I have a saw vise that is bowed and only clams in the middle)
- If you are looking at cast iron (and you probably are), inspect it for faults, cracks, etc. Many of these have been dropped in their life and cast iron does not react well to impacts.

Nice to have:

-some people line their jaws with leather to dampen vibration
-permanent mount vs. bench clamp (if you do get a clamp-on model, resist the temptation of clamping it on your actual workbench! That debris is nasty on your later woodworking and you never get it all)
-Adjustability (angle)

All of the above being said, if you happen to find a nice Disston D-3, jump on it.