Amazon is much safer but you can never be too safe with used graphics cards. You should look into getting a new 4070-ti instead as it outperforms the rtx3090 in practically every measure for the same price.
Benchmark: https://gpu.userbenchmark.com/Compare/Nvidia-RTX-4070-Ti-vs-Nvidia-RTX-3090/4146vs4081
Example New 4070ti for $800: https://www.amazon.com/dp/B0BQTSV2GG?psc=1&ref=ppx\_yo2ov\_dt\_b\_product\_details