It’s not so hard to explain:
The conditions under which transactors can use the market (repeat-purchase) mechanism of contract enforcement are examined. Increased price is shown to be a means of assuring contractual performance. A necessary and sufficient condition for performance is the existence of price sufficiently above salvageable production costs so that the nonperforming firm loses a discounted stream of rents on future sales which is greater than the wealth increase from nonperformance. This will generally imply a market price greater than the perfectly competitive price and rationalize investments in firm-specific assets. Advertising investments therefore becomes a positive indicator of likely performance.
That’s Klein and Leffler, JPE, 1981, who shy away from making the prurient explicit. This is one easy way to get a demand curve sloping upwards, namely you only trust the person if she is receiving lots of money. But what’s the point of preventing shirking if you’re going to be self-incriminating?