Avodah Zarah 13 - Buying in a Shop of an Idolater
If in a city, on the day when they celebrate their deity, some shops are decorated and some are not, then buying in the decorated shops is prohibited, while buying in non-decorated shops is permitted. One can assume that the owners of non-decorated shops do not participate in the festivities.
Resh Lakish said that this prohibition applies only to shops decorated with roses or myrtle, since they have been dedicated to the idol, and the buyer derives benefit from the fragrance when he enters the shop. The Torah only prohibited deriving benefit from idolatrous matter, saying " No part of the banned property may adhere to your hand. "
However, Rabbi Yochanan prohibits all shops, even decorated with fruit. By patronizing the shop, one provides benefit to the idol, either because the priests own the shop, or the store owner pays tax to them. If deriving benefit from an idol is prohibited, then providing it with benefit is certainly prohibited.
Art: Edwin Longsden Long - A Spanish Flower Seller