Ok to buy used hdd for server?

So there is this company that is going out of business and they are selling auctioning all of their gear to highest bids. So far no one has bid on their stuff. They are kinda unknown and it's only in person pickup with bids being online.

My question being, if I can get shitloads cheap and some maybe even free. Is there any downside? I have no idea of usage. As long as I running RAID I should be "safe" enough? Just buy as many as possible and have a stack of disk as backup as they fail?

What would recommend for building my first storage server?

*Edit*
Mainly for learner project and getting started.

Currently 5 x Samsung SSD EVO 250GB for 18$. or 16 x Origin 128 SSD for 7$ if I started with SSD. Or 10 x HGST SATA HDD with 500gb for 18$ aswell.