I've come across a few of these BIOS listings on eBay while looking for extra 5600xt cards.
These listings all seem to have solid feedback ratings. I'm just struggling to find any content (Youtube videos, Reddit threads etc) discussing this BIOS flashes. Especially when they're offering sizable jumps in MH for the 5600XT/5700xt
For example
https://www.ebay.com/itm/265008071618?ul_noapp=truety for your help/insight
I doubt they are doing anything special with your BIOS, probably just sending you one of the many already available BIOS on the internet depending on the Brand and memory. It might be worth the money if he/she is willing to hold your hand and get it dialed in if you have no idea what you are doing. I would avoid letting some random person Teamviewer into your PC though.