I’ve been convinced to build my own NAS again instead of going with a Synology and I’m thinking there are probably parts that don’t matter if you buy used vs new but I’m curious what your thoughts are?
Motherboard, CPU and RAM - no problem at all (more accurately, problems are easy to spot with diagnostics and they shouldn’t wear out).
Chassis - a bit of a wild card. The backplane in one of my systems is faulty.
PSUs - ideally new.
HDDs - almost all of mine are secondhand. Enterprise- or NAS-grade drives should have many years of life left. Ideally buy new to benefit from warranty but my experience has been great.
SSDs - nope. Buy new. I bought some secondhand Samsung SSDs and they developed problems, both threw IO errors after a few weeks. SSDs are cheap enough not to bother with secondhand.
Everything else I bought used, including the rack. In fact, the only things I bought new in my entire homelab are my router and WiFi AP.