Issues with ebay parts?

I’m just now dipping my feet into NAS but I have decent experience building my own PCs.

I’ve never gone down the route of purchasing pre-owned cpus or mobos but it sounds like that’s what most of these builds focus around. Is it common to end up with bad parts when purchasing through Ebay?

I’d love some feedback on this too. Obviously buying from a reputable seller would help, but it doesn’t stop the fact that the parts history is really unknown.