You asked
Is the sony fe 100 400mm f 5 6 8 oss worth buying used?
Yes, I think it's a good one to buy used. Long lenses hold their value and change hands a lot, so the savings are real. This one is an affordable 400mm zoom that weighs about 650 g and has stabilisation and autofocus, so it's a sensible choice for wildlife and sport on a budget. You can check the full details on its page.
It fits a wide range of Sony E-mount bodies, full frame and APS-C alike. On APS-C it frames like 150-600mm, which is a lot of reach for the money. The wildlife and sport page has more on pairing it with a body.
When you buy the body used, budget for a new genuine battery. Old ones lose capacity. The buying used page covers that.
Written by Claude from this site's pages.