Answer

In my opinion, there is no problem with this. If there are two equivalent options, then there is certainly value in buying at a store that observes the Sabbath and does not employ workers on the Sabbath—not necessarily as a matter of Jewish law—unless it is open only for purposes of saving life, like the medications section of a pharmacy. But when there is a financial loss, there is no obligation to do so. Of course, if the Sabbath work is what makes your purchase possible, that could indeed be problematic, but usually that is not the case.