My general rule is if I think it could be funky I won't buy it used. That said, I bought my boots (lightly) used and have had no problems.

As far as mandatory NEW gear I would say:
Base layer (long underwear)
Gloves/Mittens
Sock
Sleeping bag
Carabiners
Harness
Prussiks (or cord)
Ascender
Helmet
Booties
Balaclava

Have a blst. I did Alpine Ascents 7-day mountaineering in the Cascades and thoroughly enjoyed it; worth every penny.

I avoid buying any load bearing climbing gear unless i know it`s history.Anything else can be cheked ok by a visual inspection.Try to find boots that fit!

If it's going to possibly hold my weight, it has to be new. This is pretty much just hardware (carabiners, protection) and software (rope, slings, harness). Anything else, I'm fine with used. Then again, your feet might not be as forgiving as mine, in which case you'll want new boots.

As a rule of thumb I don't buy ANY climbing gear whatsoever used, especially things that your life may depend upon, ropes, protection, helmets. You just can't be sure of its history.

Half of my rack, screws, axes etc are all bought used!

No ropes, slings, harnesses though... never!

Get em from friends that I know etc. Just check out the gear before ya pay for em.