You asked
Is the sony a6100 worth buying used?
Yes, I think the a6100 is a good buy used. It came out in 2019, so it's an older body, and that's exactly the kind of camera buying used is about: an older body still gives you most of what matters for a fraction of the price. It has a 24.2-megapixel APS-C sensor and the a6400's autofocus in a simpler, cheaper body, so you aren't giving up much on the basics.
Two things to check before you buy. The body has no stabilisation, so pair it with stabilised lenses for slow shutter speeds. And old batteries lose capacity, so budget for a new genuine one. The Sony a6100 page lists every lens that fits, and your first camera has more on choosing.
Written by Claude from this site's pages.