Find out if the component has any issues before you buy it, its on the 2nd hand market for a reason.
Get the sellers contact info so that if there is any issue you werent made aware of, you can give it back for hopefully a refund. Laying down some kind of agreement to this end would help. Have it on paper or email so they cant try to back out of it.
Dont buy old components (as in released 2+ yrs ago), chances are you could afford better stuff on your budget new.
Dont get PSU's and HDD's from the 2nd hand market, these components degrade the most over time and are fairly important.