When buying a circular sock knitting machine, you need to consider what type of yarn you are going to be using with it; if you just want to make socks for yourself, or if you plan to use it for production knitting; if you want one that will do ribbing or just a hem top.
You also will need to do some research on what parts normally come with the machine.
The numbers of the cylinders correlate with the number of stitches around the sock will be. For instance, a 72 slot cylinder would make a sock with 72 stitches around. A matching ribber would be a 36 slot ribber. You would be able to do a 1 by 1 rib with this combination of ribber and cylinder, plus 2 by 2, etc.
Many people buy more than one sock knitting machine so they don't have to adjust it for different socks and cylinders. It's much easier to have a machine set up for a 54 slot cylinder and use a sport or thicker weight yarn on that one and then use another machine with an 84 slot or 72 slot cylinder for a lighter weight yarn.
If you are planning on doing production knitting, it is better to get a sock machine with a larger crank wheel so you don't have to turn the crank as often. The auto knitters, Legare's, Imperia's, etc, have crank wheels that are 1 turn per round of the sock.
If you check online for manuals, you can find pictures of the parts that are important to have with your sock knitting machine.
Also, check to make sure that none of the parts you purchase are warped, cracked or swollen. This is quite a common occurrence on some of the old auto knitters that were stored in poor conditions.
Most all of these antique sock machines will need cleaning and will benefit greatly with the purchase of new needles. When taking it apart, it is a good idea to take pictures of the machine so you know how to put it back together again after cleaning and checking it out. And please be sure to oil them, too! These machines love oil. A good type of oil to use would be 20 or 30 weight.