In no universe was that card ever worth $900.
A Founders Edition GTX 1080 has
Base Clock: 1607
Boost Clock: 1733
The MSI GTX 1080 Gaming X, according to this review:
https://www.tomshardware.com/reviews/best-nvidia-geforce-gtx-1080-graphics-cards,4725-8.html
Base Clock: 1709 (an increase of 6.35%)
Boost Clock: 1848 (an increase of 6.64%)
So, the seller trying to compare it to some kind of list price of $900 is ridiculous . . so that $530 isn't really much of a bargain. That MIGHT have been a suggested list price in 2016.... when the GTX 1080 was a $700 or so card, and the GTX 1080Ti hadn't been released yet. Even back then, $900 would be overpriced for that card.
Simply compare it to other GTX 1080 cards of reputable brands, and work from there. Also, consider comparing it to RTX 2070 cards, which perform about on par with a 1080, maybe slightly better. Pay no attention to the fact that it's specifically the "Gaming X" version with a bit of an overclock, or that some fantasy-list-price is shown.