You cannot get out of a Costco or Sam's Club without being stopped at the door, show the receipt, and wait while the clerk checks what you just paid for 100 feet from the door. Your also checked out at Best Buy trying to get out the door also.
At Costco and Sam’s, you sign a membership agreement which I believe allows them to perform the search. At Best Buy, you can just walk on by the receipt checker, as you have entered no such agreement.