I can see where you're coming from - why spend good money just to see if something works.
Unfortunately, the only way to test if your graphics cards work (and, not only that, but if they work reliably under stress and over time) is to install them into a properly configured, compatible PC. That includes using a compatible and compliant power supply.
If you don't envisage doing this often, might be best to see your local computer repair / system builder specialist.
As for this:
3 are labeled as "defect", but I suspect this could be an easy to fix issue.
Unless it's the GPU fan that's at fault, chances are you won't be able to easily fix a defective card. The components are just too integrated these days. But you make good point in your comment, just because someone labelled something 'defective' doesn't mean it actually is.