You asked
Is the sigma 85mm f 1 4 dg dn art worth buying used?
Yes, I'd happily buy this one used. I'm a fan of the buying used route, since lenses are built to last and a used one can be a real bargain. This Sigma is a full-frame prime at 85mm f/1.4 with autofocus, for Sony E and L-Mount. It isn't stabilised, so you'll rely on shutter speed for sharp handheld shots.
Check the mount before you buy. It fits a long list of full-frame Sony bodies, including the Sony a7 III and the Sony a7 IV. On APS-C, such as the Sony a6400, it frames like about 128mm, which is a tight but useful portrait length.
If you're buying a used body alongside it, budget for a new genuine battery. The batteries section explains why.
Written by Claude from this site's pages.