There is used gear, and abused gear, and there is sight unseen stuff. Shopping for used gear is a risk, The item might be from an out of business company that you can never find repair parts for. You might be buying an item, discontinued, and no parts available, how, as a beginner, are you to tell?
Herman's advice above is written better than I could. Until you have a few years experience, the risk of throwing away your money is too great. With one exception, dive shops can buy at a big discount, Regulators, BC's and gauges, if they are used in rental for an agreed time period. My favorite dive shop, buys new rental gear every year, then in the winter, rebuilds every piece and sells it used to his students. . . ask your instructor if he can find this for you.
Herman's advice to "rent gear for a while and try different types and brands". . . is the best advice to get comfortable gear that will last you many years. Some divers buy new, at the suggestion of a dive shop employee, and spend too much, and wind up with poor fitting BC's poor fitting wet or dry suits and too complicated computers. A dive shop should be a friend to trust, many are. . . some not so well. Sounds like life.