READ THIS BEFORE BUYING:
NO - BestBuy does NOT offer the truly 'factory unlocked' iPhone X which Apple now offers. They did initially, for an added $100 fee, but quickly stopped selling it.
It is important to understand the benefit to buying the fully unlocked iPhone X vs. paying outright for an iPhone X tied to a particular carrier.
The Factory unlocked iPhone X will work on both CDMA and GSM networks, period.
The AT&T version is only going to work on GSM networks, and the Verizon version only on CDMA.
If you pay outright for the AT&T version, will it work on most European GSM networks? Absolutely. But it will not work on Verizon.
Personally, I bought the AT&T iPhone X and paid full price. Once Apple began selling the fully unlocked versions a week later, I returned my AT&T iPhone X and bought the unlocked version. Who knows if I'll want to switch to Verizon in a year.