With used hardware, it very much depends on what the hardware is and what you’re using it for. if you can find something from the last 10 years it’s probably worth it, but I wouldn’t get anything older than that. Power usage is the main concern, as systems have been plenty powerful enough for most applications for sometime. Hardware reliability would be another factor.
When I was looking a couple of months ago, it looked like $200 USD was the sweet spot for used hardware, but at that price point, you could get one of those NUC knockoffs brand new, such as the Beelink N100. It just depends on what you need.