Reciprocity refers to a social norm in which we feel a sense of obligation when receiving positive actions towards us. For instance, receiving a favor or gift can trigger feelings of indebtedness to repay the act in someway. While reciprocity allows one to build relationships and exchanges, it can be exploited to gain someone's compliance to a request. This is because feelings of obligation can be felt even when receiving a positive action we do not want.
Before attempting to sell something, a salesperson may give you a small gift (e.g. a pen or a free sample). Even if this gift is unwanted, receiving it can still trigger a need to repay the gift, usually in the form of purchasing whatever is being sold.
A common compliance technique is to start by making a large request that's likely to be rejected followed by a second, smaller request. The second request, which is the intended request, is more likely to be accepted as it's seen as a favor.