Please help me with my pc

Everytime i put my gpu under load, something happens. I've gotten artifacting, full pc crashes, or just game crashes. It runs extremely solid as it should for about 10 seconds and then just stops. Also, it keeps not booting and lighting up the vga light like the graphics card isn't connected, and i don't know what to do. It's seated correctly, i ran ddu and installed new drivers, i enabled and disabled docp, the power supply is good, and everything is fine until i open a game. I downloaded all my games and sat in windows for like 2 hours no issues. I don't wanna believe it's a faulty gpu, but i'm starting to think that's all that's left. i have a rtx 4070ti that i bought used but it said "Tested and fully working with no artifacts." and it's from a reputable seller on ebay. Pls help.