I do both. I replaced a new amp with a pair of used ARC monoblocks, and I replaced those with a new Cary that I am still running. I replaced a new pre with a used Cary, and I replaced that with another used Cary. I replaced a three piece digital front end that I put together with new pieces with a Marantz that was a "factory refurb" (so I guess that counts as used). So far my speakers have all been new, but I would consider used on those as well.
New vs uses in your current setup
Curious the ratio new to used/demo in your current system comprised of.
I’m equally split on a “per box” basis although I tend to look for used cables almost exclusively.