Are these pots suitable to cook in?
These are enamelled pots - perfectly fine and intended for cooking, albeit a bit sensitive to chipping if not handled carefully. You can find various listings of that exact set on the Internet, e.g here or here.
Enamel is a hard, glass-like, non-porous substance and pretty non-reactive. As long as you are not exposing it to extreme temperature changes (when it can crack like glass) or excessive scrubbing, it remains pretty much the same even with prolonged use. So buying them secondhand is a good investment, if they are in a good condition. Do not use enamelled pots with chipping, less because of the rust from the iron core, but because in that case there’s a (small) chance of enamel shards or splinters ending up in the food. With good care however, enamel pots can be used a long time.
Note that the surface is quite hard and scratch-resistant, so not bad per  se and reasonably well to clean. It’s not non-stick, though, so you will want to use a bit of fat if you intend to fry or roast something in them. Some cooks like the light inside color, because it makes it easier to judge the color, e.g. when browning onions.