The real issue is that new divers don't know what they don't know yet. Buying used gear, especially in a grab bag sale, is likely to result in some unusable or redundant dive gear.
Yeah, but for a new diver who doesn't know what they don't know yet, buying new gear is
also likely to result in some unusable or redundant dive gear, at 3x the price to their LDS.
If you are comfortable with your own assessment of technical equipment, evaluating risk, etc., then
you can probably tell in a few seconds, or a quick pool session, whether some used fins & mask fit well, whether a BCD holds air, etc. Instead of buying used, you could buy new gear and pay your LDS the new-equipment premium to advise you on purchases. Sometimes it's worth it for the education & to support a local business, sometimes it isn't...
Tanks & regulators and hoses need to be inspected & serviced or replaced -- but that applies whether they are 2 years old or 20 years old. [Yeah, Atomic has a longer service interval than most, and yeah well maintained gear with no IP creep may not need any service...but that goes under the headings of "inspected" and there's nothing to stop a new diver from also getting training and servicing a regulator.]
If the used gear is in the right range (ie., the size tag makes you think it would fit, it isn't stinky & crusty & dirty & full of holes, etc) and the right price for you, then I say give it a shot.
In a year (or 50 dives) you'll have a better idea whether that stuff is right or not. If you discover you really want different equipment, it will be easier for you to stop diving $500 of used gear that you don't like than to keep using $1500 of new gear that you don't like.
Me, I'm diving a motley collection of used gear, and enjoying it.