This is an interesting question, because government regulations have a tendency to favor large corporations. For example, a million dollars worth of regulatory compliance to open a store is nothing for wal-mart, they wouldn't even blink an eye. But for mom-n-pop corner grocer, it would shut them down immediately. There are also zoning regulations that strictly limit where people can sell and compete, and what they can sell. Wal-mart can also have the luxury of having a small army of lawyers, litigators, and tax accountants on their full time payroll that would kill any other small business.

I think wal-mart also gets some favors sometimes like eminent domain, and road infrastructure around the store area. However, I don't really put much weight on this, because they still pay billions in taxes per year. IMHO, the state takes far more from them than what wal-mart gets in terms of free benefits. Not to mention, many of the infrastructure services and regulations are the same for any business. There are also things like government food-stamps, but those are also accepted by almost any grocery store.

I often like to think of libertarian as having small and decentralized roots, but at the same time, economies of scale matter. They matter a lot. In a free society, I still think we would have large chains like wal-mart. Also, it's clear they cater to the private sector and not the government sector. We're not talking lockheed martin here.

The US also has chains like Target and COSCO, that compete head on with wal-mart and do ok. Safeway corp, Albertsons corp, also have 1000's of locations inspite of wal-mart. So it's not like wal-mart has a total lock-in.