What should i know before buying from a computer swap meet?

With the inflation in prices of HDDs everywhere, i was planning on purchasing storage at my local computer swap meet. I don't know if i can trust these guys too much, is there anything i should try to remember?

Also, what are the HDD hierarchies? For instance, samsung spinpoint is number one, expect it for (x), while seagate barracuda is approximately (y). Go for the samsung if it is less than (z). Anything like that i should try and keep in mind guys?

Any help would be much appreciated :)