This post is a reaction to another, on gift economies in D&D, by Homicidally Inclined Persons of No Fixed Address, who is well worth a read. 

There are a few clarifying points to be made on the subject: 

1. A "gift economy" is a kind of economic structure that the West doesn't really use anymore, but it was probably the most common kind of economic structure for most of human history
2. "Gift Economy" is a bad term. In a gift economy, every "gift" imposes a debt. In fact, you can generally impose a debt on someone involuntarily by giving them a gift. I feel that HIPoNFA was fully aware of this, but didn't include it in their article because to them it is too obvious to be worth stating clearly. 
3. Gift economies are not exclusive with market economies. The same person can engage in gift economy behaviors with some people at some times, and market economy behaviors with other people at other times. Because gift economies are based in debts, you would only engage in gift-type exchanges with people that you expect to be able to collect debts from. There are three components of this that are important: 
   1. You expect to see them again 
   2. You believe that they will be able to repay that debt
   3. You believe that they will be willing to repay that debt
4. The way in which some societies focus on honor (there are multiple forms of honor, not all of them are this one) is caused by (3a) and (3b). Most human societies, including ones with strong states, have not considered it to be the job of governments to enforce contracts between individuals. When this is the case, it becomes important for individuals to only make such contracts (debts, including gift-based ones, are here considered to be a kind of contract) with people that are likely to honor those contracts. If someone has a known history of not repaying their debts, even on accident, you will be less likely to let them go into debt to you. Let me give some examples: 
   1. Alice gives Bob a sheep that she doesn't particularly need, with the implied social norm that Bob will then have to give Alice things of equal value at a later time. That sheep is then publically stolen from Bob by Carol, and Bob does nothing to stop Carol from keeping and enjoying that sheep. When Alice needs something from Bob next season, Bob is too poor to give Alice anything of significance. Bob's honor has decreased, because he is now considered to be someone whose wealth can be freely taken, so he cannot repay debts that he enters into. Carol's honor has also decreased, because she is the sort of person who breaks rules like "don't steal". If Carol is willing to break rules such as "don't steal", then she's also probably willing to break rules like "repay your debts". 
   2. Alice gives Bob a sheep that she doesn't particularly need, with the implied social norm that Bob will then have to give Alice things of equal value at a later time. Alice comes by Bob's hut next season, and observes his many pigs. She says to him "those are very nice pigs. I wish I was a good enough pig-farmer to have pigs like that!", with the implied social norm that Bob will understand that this is an implied request for a pig. Bob knows this, because he understands the basic etiquette of his own culture, but says "yep. You sure are shit at animal husbandry. Anyways, maybe with more practice you can git gud and be an epic gamer pig-farmer like me". Alice, incredibly insulted and realizing that Bob has no intention of repaying his debt to her, complains to everyone that Bob is an asshole, a braggart, and a cheat. Bob's honor has decreased, because everyone now knows that he won't repay his debts even when is fully capable of doing so. 
5. Due to (2) and (3), if you give someone enough gifts, they will owe you enough that will either need to accept you as their social superior and begin doing whatever you say (within reason, I suppose) in order to repay the debt, or reject doing that. If they choose the latter, everyone who knows that they have done so will understand them to be generally willing to break social rules, and as such have no honor. If someone has no honor, no one will let them go into debt to them, which means that they will effectively be excluded from all the material benefits of society. As such, giving a big enough gift to someone forces them to choose between either acknowledging you as having power over them, or becoming a hermit. 

I assume that most people in most small villages that the PCs encounter in a D&D game are using something like a gift economy, or are at least running tabs with each other -- in much of middle ages Europe, this was how things worked. However, this is never relevant to the PCs because the PCs are roaming strangers who use violence to solve their problems -- any smart peasant does not expect to see the PCs again, and knows that even if they did, the PCs have no strong social connections and might be willing to deploy incredible amounts of violence against them. As such, the PCs need to pay for things immediately, using the vast amounts (relative to the average peasant) of wealth they have. 

While the average person will not let you go into debt to them, there is still another way to enter the gift economy: have other people go into debt to you. 

Putting this into D&D terms, in a gift economy, two things should be true: 

1.  if you give a reasonably sized gift to someone in a village (such as say, dealing with their monster problem) they now owe you, and you get to access that village's gift economy, rather than always having to pay in cash. The gift economy mechanics can be most easily represented by the villagers letting you run up a table proportional to some of reputation stat (the simplest way to do this might be gold pieces equal to your level times your Charisma score, but surely you could make it more complicated) 
2. there should be an amount of gold pieces (it should be ridiculously high) that you can pay to an NPC per level (or hit die) that lets you make a Charisma check or the like. If the check succeeds, the NPC is now your hireling. If the check fails, the NPC is now excluded from society because they refused to be your hireling.