Arrangements where agents commit to buy only from selected vendors, even when there are more preferred products at better prices from other vendors, are common. Consider local currencies like “Ithaca Hours”, which can only be used at other participating stores and which are not generally convertible, or trading circles among co-ethnics even when trust or unobserved product quality is not important. The intuition people have for “buying locally” is to, in some sense, “keep the profits in the community”; that is, even if you don’t care at all about friendly local service or some other utility-enhancing aspect of the local store, you should still patronize it. The fruit vendor, should buy from the local bookstore even when her selection is subpar, and the book vendor should in turn patronize you even when fruits are cheaper at the supermarket.
At first blush, this seems odd to an economist. Why would people voluntarily buy something they don’t prefer? What Mailath and his coauthors show is that, actually, the noneconomist intuition is at least partially correct when individuals are both sellers and buyers. Here’s the idea. Let there be a local fruit vendor, a supermarket, a local bookstore and a chain bookstore. Since the two markets are not perfectly competitive, firms earn a positive rent with each sale. Assume that, tomorrow, the fruit vendor, the local book merchant, and each of the chain managers draw a random preference. Each food seller is equally likely to need a book sold by either the local or chain store, and likewise each bookstore employee is equally likely to need a piece of fruit sold either by the local vendor or the supermarket; you might think of these preferences as reflecting prices, or geographical distance, or product variety, etc. In equilibrium, prices of each book and each fruit are set equally, and each vendor expects to accrue half the sales.
Now imagine that the local bookstore owner and fruit vendor commit in advance not to patronize the other stores, regardless of which preference is drawn tomorrow. Assume for now that they also commit not to raise prices because of this agreement (this assumption will not be important, it turns out). Now the local stores expect to make 3/4 of all sales, since they still get the purchases of the chain managers with probability .5. Since the markup does not change, and there is a constant profit on each sale, then profits improve. And here is the sustainability part: as long as the harm from buying the “wrong product” is not too large, the benefit for the vendor-as-producer of selling more products exceeds the harm to the vendor-as-consumer of buying a less-than-optimal product.
That tradeoff can be made explicit, but the implication is quite general: as the number of firms you can buy at grows large, the benefit to belonging to a buy local arrangement falls. The harm of having to buy from a local producer is big because it is very unlikely the local producer is your first choice, and the price firms set in equilibrium falls because competition is stronger, hence there is less to gain for the vendor-as-producer from belonging to the buy local agreement. You will only see “buy local” style arrangements, like Ithaca Hours, or social shaming, in communities where vendors-as-consumers already purchase most of what they want from vendors-as-producers in the same potential buy local group.
One thing that isn’t explicit in the paper, perhaps because it is too trivial despite its importance, is how buy local arrangements affect welfare. Two possibilities exist. First, if in-group and out-of-group sellers have the same production costs, then “buy local” arrangements simply replace the producer surplus of out-of-group sellers with deadweight loss and some, perhaps minor, surplus for in group members. They are privately beneficial yet socially harmful. However, an intriguing possibility is that “buy local” arrangements may not harm social welfare at all, even if they are beneficial to in-group members. How is that? In-group members are pricing above marginal cost due to market power. A “buy local” agreement increases the quantity of sales they make. If the in-group member has lower costs than out of group members, the total surplus generated by shifting transactions to the in-group seller may be positive, even though there is some deadweight loss created when consumers do not buy their first choice good (in particular, this is true whenever the average willingness-to-pay differential for people who switch to the in-group seller once the buy local group is formed exceeds the average marginal cost differential between in-group and out-of-group sellers.)
A Fine Theorem 
Buying locally may not make sense to economists, because their way of looking at the world leaves “remaining no other nexus between man and man than naked self-interest, than callous “cash payment,”” as the Communist Manifesto put it. For a different view of what economies should do, see the Pope’s recent encyclical, “Laudato si.
Does the analysis hinge on that the supermarket and the chain bookstore stubbornly remains to operate in the area? If they find it unprofitable they might leave, and you are worse of, no?
I buy locally because I know the sellers are not selling me snake oil. For instance I know the local farmer treats his animals and workers better than the big organizations and I know that the reputation of this farmer would be damaged a lot if people found out he wasn’t treating animals and people as well as he claims. There is no possibility for me or anyone in the community to find out how bigger farmers from outside treat their animals – and if someone found out they were cruel I wouldn’t find out immediately like with the local farmer.