I don't know what price to expect for used machines, but I can't imagine you should pay more than $100 or so for a good used machine.

For basic sewing and repairs, avoid machines with lots of attachments and fancy stitches. All you need is a good straight stitch and zigzag, but a handful of extras like stretch stitch, satin, and a buttonholer are also handy. Avoid smaller "half-size" (or whatever name they market them under) models, these are not only entry-level but they are far less sturdy, reliable, precise, or powerful than full-size machines. They don't have enough power to sew through thick or layers of fabrics, like hems of jeans or messenger bag straps.

Check if there is a sewing class offered at a local store or school, and what kind of machines they use. They are generally very sturdy, basic machines and can be had for reasonable prices, even new retail.

If you're looking at a used machine, check all the contact points for wear. There are a LOT of them, but most are visible by opening the front or side panel. These are the places where metal parts touch other metal. The only regular maintenance a sewing machine needs is oil on the moving parts, basic cleaning, and keeping it properly adjusted (tensions, etc.) and it will run forever. There will be visible wear on the moving parts if the machine has been neglected, it will make clicking or whirring noise, though noise might just mean it needs to be lubed.

ETA: for what its worth, my machine is a Brother Pacesetter. It was a gift from my parents several years ago, and it's a champ. I use it quite a bit for making clothes, alterations, and repairs. It's not strong enough to do really heavy duty sewing, but it had no problem sewing straps on my mess bag. It's really precise, too, which is important for thin or stretchy fabrics.