You are not obligated to pay if you sit down for an impromptu makeup session with a salesperson. And you aren't obligated to buy anything if you are paying for a makeup application. You are, however, expected to buy something if you booked a makeup consult that's free and you were pleased with the results.
Consider how much of the salesperson's time you took. If you asked for a consult and took up a lot of her time asking questions, then you should consider buying something as a courtesy. But if she nabbed you to try a liner or lipstick and you didn't like it, thank her and move on.
