What is the best used camera lens to buy?

Best used camera lens for most people: Canon EF 24-105mm f/4L IS USM, if you shoot Canon DSLR or can adapt EF lenses to your mirrorless body. It is a practical used buy because it covers wide-to-short-telephoto everyday shooting, has a constant f/4 aperture, and is widely available on the used market.

If you mainly shoot portraits, get the Canon EF 85mm f/1.8 USM instead. If you use Sony full-frame and want a pro telephoto zoom, the Sony FE 70-200mm f/2.8 GM OSS II is a stronger but much more expensive used-category pick.

Before buying any used lens, check for internal scratches, fungus/mold, heavy dust, working autofocus, smooth zoom/focus rings, and a correctly functioning aperture. Also verify mount compatibility with your camera body.