I’m currently a student so I don’t have thousands to spend on hard drives and I’d really love to make a compact storage server with large capacity to do stuff like a Plex server. I’ve been looking and I see good deals on sites like eBay, however they seem too cheap sometimes. What are things I should look out for/should I just not get used hard drives?

I need to mention this would not be for critical data such as personal photos etc. it would just be raw data storage that I could afford to lose- it would just be a pain.