As a general rule I do not like buying second hand electronics, be it ASIC, GPU, rig or laptop, ...
Second hand mostly means no warranty and that is if you receive something that works, are not fake, ... in the 1st place.
Very true.
I think that's the first and most important point to keep in mind.
Other than that searching locally, I can have access to gaming PCs second hand that some boys sels because they need money, and theyr cheap.
I guess that if I get good deals I can lose warranties but earn economy.