buying an used 1080ti

[MiRai]

Would you buy an used 1080ti

If I didn't already have two, yes.

how would you test it

Play games with it like you normally and turn up the video settings to put as much load as you can on the thing. If it buckles, the it may be bad, but it may also just be overclocked and need to be down-clocked a bit (or just kick up the input voltage a notch).

what's a good price for it?

It's essentially an RTX 2080 with an extra 3GB of VRAM, but without the ability to handle ray-tracing. So, I probably wouldn't pay much more than what an RTX 2080 costs.

should I try to get it from a known brand like Asus or Gigabyte ?

Both EVGA and MSI have transferable warranties, whereas neither Asus nor Gigabyte do.