Monopolies are de-facto government bodies. It isn't that Ebay is administered by USG (although I'm sure Ebay has an NSA/DHS detachment); it's that it makes no difference to the user whether a government bans the sale of books on online flea markets or the only practical online flea market does.

If the only store within a 2-hour drive is walmart, walmart is your equivalent of the soviet centrally-planned economy.