Built a gaming desktop for a friend almost 2 years ago; used GPU and CPU (maybe a few others things too), everything's going great. It helps that our local Cragslist offers efficient buyer protection.

Server-side, I also bought used Xeons for an old box and recertified 10TB Exos. No issues there neither.

The HDDs are a bit of a gamble, but for anything else I can only encourage you to buy used!