You asked
Is the sigma 50mm f 1 4 dg dn art worth buying used?
Yes, I think it's a good candidate for buying used. Lenses are built to last, and a used copy of a well-regarded prime usually gives you most of the lens for less money, as the buying used guide explains. I can't tell you current used prices, because the site doesn't track them, so compare a few listings before you commit.
Check the mount first. This 50mm is made for Sony E and L-Mount full-frame bodies, and it isn't stabilised, so you'll want to think about that if you shoot handheld in low light. On APS-C it frames like a 75mm.
If you want a different focal length from the same Art line, the Sigma 85mm f/1.4 DG DN Art is the other prime I'd look at. For the full lens details, see the Sigma 50mm f/1.4 DG DN Art page.
Written by Claude from this site's pages.