have 3 days to test it until payment is released to the seller. I may not be able get a PSU
the minimum requirement i've seen for the RTX 3070 Ti is 650w.
while you could test it with a 550w, you'd never know if it can handle it's own max core & memory output this way.
if it's new just keep it.
the manufacturer will handle any later malfunctions through RMA.
if used; contact the seller and explain the situation.
maybe they'll extend your coverage for a couple days.
or take it to a local shop and have it tested.
they should have test-bench systems ready for use.