The best gift to compel reciprocity is of course printed return address labels. You can probably think of many more examples where you have felt obligated or compelled to do things for others because they have done something for you. Clearly, relationships are built on reciprocity. Difficulties occur when reciprocation becomes manipulation. It is not bad to give gifts to others or accept gifts from others.
Salespeople frequently use the contrast principle because it is so effective.
People tend to remember the last thing they saw or were told much better than something they were shown or told earlier. If the last home, in this case, is a great deal nicer than the first, then the colorful memory of the nicer home will make the less expensive home seem drab and dull. Dessert is cheap compared to the expensive meal you just have. Fast food customer service reps will take your order, then mention an additional item. The customer service person asks you to buy the cookies which you would not have purchased due to your weight control program you are on.