Background: To prevent alcohol availability through retail, legal age limits forbid the sale of alcohol to minors in many countries. This study tested the effects of minors’ alcohol-purchasing strategies and knowledge of compliance with age-limit legislation on the availability of alcohol through retail in the Netherlands. Methods: Making use of exploratory, field-experimental design, eight 17-year-old minors participated as mystery shoppers over the period of one weekend. The mystery shoppers were allowed to use their own strategies to buy alcohol. The experiences of the first group were shared with the second group. Results: Out of the 134 times, the shoppers entered an outlet to buy alcohol, and 119 attempts were successful (11.2% compliance). A total quantity of 225.78 US gallons of alcohol-containing beverages was bought. The first group bought 53.93 L on their final day, and the second group bought 53.92 L on their first day, making use of the strategies of the first group. Conclusions: Current self-checkout lanes in supermarkets produce extreme alcohol availability and are even less effective than traditional checkout lanes: Policies for age verification fail. This first small-scale study shows that underage mystery shoppers learn quickly and are able to buy more alcohol with each subsequent visit.