Let's say a guy (let's call him Rene) found an old chest in his new house. The chest contains only a strange bottle filled with 600 milliliters of a pink fluid that emanates an eerie light. The note says that it is a love potion, preserved from ancient times. The last of its kind and it cannot be replicated by technological means. Only 10 ml is enough to take the heart of the opposite sex forever. This guy tries the potion on his cute and young roommate and it works.
Now my character is married and he wants only money. 590 ml of this wonderful potion remains. How does this guy sell the potion at high prices (above 100,000 US dollars) and make potential costumers trust him without wasting the precious liquid on demostrations?
Note: The effect breaks if the target knows that they drank a love potion. The target falls in love with the first person she/he sees in the first hour.