He countered with, "Well I bought the food for the gecko this week, I don't see why i'm in any debt to you". This is where the web of favors become complicated. Favors can be easily traded, but are very hard to determine the non-monetary value of a favor. Does getting up early in the morning to pick someone up from the airport cost more than paying for a meal at a nice restaurant (imagine that the expenses equal on both sides); which one costs more and who's in the bigger favor debt to repay the other?
Although they may be difficult to classify and are different for each individual, they are just simply amazing. Think of all the favors you have done in the past for anyone. You weren't expecting them to just hand you some change or a few dollars in order to repay the favor. They repaid you in kind with a similar favor they thought you would appreciate. If you were to pay for your friend's dinner and then in turn they give you a mint as repayment (to make this clearer, if they also stated something along the lines "Hey thanks for buying dinner!") would you not be willing to trade favors or be as friendly to that person?
Businesses often use these sort of favors known as accounts receivable. Basically saying we know how much the expenses are and we will pay you, you just have to trust us (rather than signing a legal note saying they will instead). Without using any court system they developed a reasonable state of trade where both businesses generate wealth by trading with each other on the terms of a favor; and like in real life, if that trust is misplaced that company will never do business again with the other. Of all the trades I can think of that would be the most intricate, versatile, and most difficult to appraise would be the relation of favors to others.