What are the causes of a change in demand?

Janette goes into an electronics superstore to buy a new computer. the salesperson works with her to find the right model, and she agrees to buy a new computer for $500 and a floor-model 18-inch monitor for $100. just when she is ready to pay, the salesperson walks away for a minute and then comes back to inform her that the monitor she was ready to purchase can't be sold for < $125. she agrees to the extra $25 even though they had initially agreed on the $100 selling price. janette has fallen for the technique of gaining compliance.