Quoted for relevancy.

None of this happens in a vacuum. Stores aren't a perpetual motion machine, driving the BTC price down every time we purchase from them. In order for them to have BTC to depress the price of BTC there need to be prospective purchasers purchasing BTC and driving up the price.

I doubt there'll be 100% efficiency - some purchasers will have a stash of BTC and won't bother buying more simply because they want to buy something, and some merchants will keep some or all of their BTC. But for the moment at least most purchasers will buy BTC around the time they make a purchase, and most merchants will sell BTC around the time they make a sale.