Is used fencing gear safe to buy?

Most of it, yes, with one loud exception. Never buy a used mask sight-unseen, and be skeptical of used masks generally: mesh fatigue is invisible, a dented or rusted mesh can fail exactly when it matters, and a mask that's been through unknown years of hits is the one piece of gear whose whole job is stopping a broken blade from your face. Masks are also the cheapest safety-critical item to buy new. Buy the mask new.

Genuinely fine used: jackets, breeches, plastrons (inspect seams and fabric for thin spots or repairs), lamés (test conductivity if you can, dead spots are the common failure), gloves (check the palm), body cords (test or budget to re-tip), and weapons (blades have life left or they don't, look for deep kinks and rust at the bend point; grips and guards last nearly forever).

Where used gear comes from: club swap bins and older teammates are the best source, gear with known history, often free or nearly so, and outgrown kids' sizes cycle constantly. Club listservs and fencing-specific resale groups beat general marketplaces because sellers know what they have.

The math that makes this worth doing: kids outgrow jackets yearly, and a full used soft-goods set often costs less than one new jacket. Spend the savings where it counts, a new mask and a good glove, and nobody on the strip can tell the difference.