Simply giving somebody a compliment can increase persuasion, and the chance that the consumer will purchase the product. This has occurred numerous times in my life. For example, when I've been trying on make-up in a shop, the shop assistant may compliment me, which is likely to increase the likelihood that I will buy the make-up I tried on. Another example is when trying on clothes. Again, if the shop assistant compliments your appearance with the new clothes on, it is more likely that you will buy them.
Lots of research has focused on the effects of compliments on persuasion. For example, Cialdini (1993) looked at the tactics sales assistants may use when trying to persuade customers to purchase a particular product, for example complimenting a customer. It has been found that consumers avoid cognitive effort in decision making (Chaiken, Liberman & Eagly, 1989), and simply complimenting a customer simplifies decision making for them, increasing the likelihood that the product will be purchased.
Many studies have directly looked at the effects of compliments on the amount of products sold. For example, Dunyon, Gossling, Willden & Seiter (2010) found that when a telephone salesperson complimented customers, sales of add-on merchandise increased.
Cialdini, R. B. (1993). Influence: The psychology of persuasion. New York, Morrow.
Chaiken, S., Liberman, A. A., & Eagly, A. H. (1989). Heuristic and systematic information processing within and beyond the persuasion context. Unintended thought: Limits of awareness, intention and control (pp. 212-252). New York: Guildford.
Dunyon, J., Gossling, V., Wilden, S., & Seiter, J. S. (2010). Compliments and purchasing behaviour in telephone sales interactions. Psychological reports, 106, 27-30.