This is legit. Bought a 4090 from here. Beware the difficulties, though.
1. the die position on the card is different than the 880M/m290x, so the cooler wont fit. You'll either need to DIY one or cutom order one.
2. In my first attempt to get the car running in my Alienware 17, the motherboard died. Im guessing the gpu caused a voltage spike at boot that the old VRM's couldn't handle. Tried again with a new motherboard and got an 8 beep video error because the board couldnt recognize the card. I verified it wasn't a fake by pluging it into my pc with a riser, where it was recognized. You'll need to modify the bios.
3. The 4080/4090 willl request more power than the system is able to dish out. You'll need to limit it through afterburne or similar programs, and even then ther is risk of frying the board under load.
If you saw all of this and the prices or the cards themselves and you still want to do it, go for it. Have fun.