Daniella invited Nicole to her house for a party, which turned out to be a sales pitch for the cosmetic line that Daniella had recently started selling. Nicole didn't need any cosmetics, but since she was already at Daniella's house and enjoying the complimentary food and drink, she felt obligated to purchase some lipstick. Daniella is using the:

rule of reciprocity

Explanation:

The rule of reciprocity: In social psychology, the reciprocity principle or rule or reciprocity or the law of reciprocity is defined as a process of paying back what a person might have received from the other person in any of the social situation.

Example: If a boy named Charles favors his friend John, then his would most likely to return the favor in any of the way.

In the question above, Daniella is using the rule of reciprocity.