The best advice I can give a new diver is to go very slow at getting your own gear. Upgrading scuba gear should not be your plan, but rather a symptom of the addiction we all suffer from. Scuba gear can last most of your diving career if properly maintained. So, rent a bit, this allows you to know what you want. What type of BCD, jacket or BP/W, do you understand the difference? Where are you taking your diving career, professional, master enthusiast, technical or cave? What do you have now? You should answer all of these before us on the internet can provide our recommendations.
I have found several items on the second hand market, but you need to know what you are buying and who you are buying from because that determines the risk you are taking. Regulators keep you alive underwater, so the risk I would take with those is minimal.
As mentioned before, there are basic recommendations as to what comes first. Mask, snorkel, fins, booties are first. Then gloves, hood, and wetsuit or drysuit. Next I would suggest computer, cutting devices, lights, compass, and any/all other needed accessories you need for your particular diving. Next is BC, then regulators, and lastly if at all, tanks. Tanks are really dependent of if you do any local diving.
You can search for each item above on SB and find many articles and recommendations for each one. The used market can keep the costs down, but know who you are buying from, or have it checked by someone you trust with the knowledge to know what they are looking at. I have found that if you ask the most basic diving questions about equipment the seller either knows what they have or they don't. If they don't, go another direction until you have more experience. I will caution about buying old wetsuits, they compress during diving and over time loose the ability to decompress and the insulation value drops significantly.
I hope this helps, again, go very slow, buy once and cry once. You likely will not "need" to buy again, not to say that you won't, again the addiction thing.
Keep diving fun!