You asked
Is the canon ef 100mm f 2 usm worth buying used?
Yes, I'd look at this one used. It's a 100mm f/2 prime for Canon EF. On full frame it frames as a true 100mm, and on APS-C it acts like about 160mm. It fits a long list of bodies, including the full frame Canon EOS 6D Mark II and the APS-C Canon EOS 80D.
Used is a sensible way to buy lenses and bodies, since they're built to last. I don't have current prices here, so compare listings yourself. If you're buying a body too, budget for a new genuine battery, as the buying used guide explains.
If you want close-up work more than portraits, the Canon EF 100mm f/2.8 Macro USM is a common used alternative.
Written by Claude from this site's pages.