I tried doing that one day. I got my butt chewed out by a mom because she thought I was being a jerk to her daughter. (Keep in mind that the daughter had approached me multiple times during the same trip asking me, politely, to purchase some cookies. I, politely, answered "No, thank you" each time. So, when she asked, politely, one last time, I thought I'd provide a reasonable explanation)...
After that day/incident, I just decided to go down either the "No, thank you" or "I've recently purchased some cookies" road for the rest of my life...