You asked
Is the sigma 30mm f 1 4 dc hsm art worth buying used?
I can't tell you a used price for the Sigma 30mm f/1.4 DC HSM Art, because the site doesn't list one. What I can say is that it's an APS-C lens, so check that your body is APS-C, or a full-frame Nikon or Pentax you're happy to run in crop mode. On a Canon EF full-frame body it won't work at all. Its page lists every compatible camera.
The general case for buying used holds well here. A lens like this is built to last, so a used copy in good shape is usually the sensible way to save. My advice is in buying used. Compare sellers' prices against the going rate before you pay.
If you shoot full frame and want a fast Sigma prime, look at the 35mm f/1.4 DG HSM Art instead. It's a different lens, but it's the one I'd point a full-frame buyer toward.
Written by Claude from this site's pages.