Found a used 500gb 850 evo on amazon for 128$ plus about 7$ in tax. Is that a good deal?
No,its a pretty bad deal. For only ~$10 more you can get a brand new SSD with similar real world performance.
I would say not a great deal when a brand new one is currently less than $25 more.
It's probably fine, but you don't know how it was used, either.
How would I format it?