At this time, the Pixel/Pixel XL has an exclusive US carrier deal with Verizon.
BestBuy is selling what Verizon is willing to let BB sell, which is the Verizon-subsidized, Verizon-locked versions (in all but Verizon exclusive blue). These phones are sold with Verizon's micro-SIM card pre-installed.
There are unscrupulous third-party sellers who are abusing the Verizon subsidy by "forced unlocking" Verizon subsidized Ps/PXLs and then selling them as "Factory Unlocked."
Google Play Store is the authorized and reputable source for Factory Unlocked Ps/PXLs (in all but the Verizon exclusive blue). BB has never, to my knowledge, sold Google Play Store phones.
Unlocked Ps/PXLs can accept any carrier's micro-SIM card, but you will still have to negotiate with said carrier for actual service. No carrier is obligated by law to provide service on all possible devices. So ask your intended carrier about coverage before you buy unlocked.