I've sold old computer hardware on both eBay and hardforum [1]. The nice thing about hardforum is a lot of buyers have heatware [2] which makes it easy to judge their legitimacy. I'm sure there's other forums out there that have similar buy/sell sections you can use. To increase your legitimacy it's best to maybe explain a bit about the hardware (e.g. how hard was it run) and have a picture with something showing ownership (e.g. the hardware with a piece of paper where your username is written).

[1] https://hardforum.com/

[2] https://www.heatware.com/