Used RTX 4090 or new RTX 5090

I'm a bit torn on choosing what's best for me. Upgrading my build to AM5 with a 9800X3D cpu and was waiting for the RTX 5080, but to be honest the VRAM is scaring me as I doubt it will be that future proof.

I'm now looking into the 4090 or 5090 cards. There are some deals on a used 4090 for 1400-1500 euros near me. Used parts also kind of scare me, but these cards still have some warranty period left. So I'm looking into some advice from you guys. What is a good price for a used 4090? What should I look out for or be aware of, why would buying a new 5090 be a better choice,...

Thank you!