OPTIONAL TIP: You can find used equipment that is very good, and buy it, for stuff like fins, or compass, or anything else. Many people "try" diving but either get bored of it, get spooked by seeing a shark, or realize they have problems equalizing, or they fail the class. (Or they just need money) SO, they try to sell their equipment online at a big bargain.
Buying a BCD that’s used is risky, though, same with a regulator... but if you try them out and they work just fine, then maybe it’s ok to use/buy them.
Generally speaking, if an item will not kill you if it malfunctions, then it’s totally cool to buy it used. But stuff like cave diving lights, (or really any cave equipment), or regulators, or BCD’s, you probably want to buy them new unless you are really strapped for cash