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.
They're a publically traded monopoly marketplace with special legal protection from being sued. I would be fine with them being forced to act like what they pretend to be- a commons, a market.
The freedom of a giant, monopolistic company is not aligned with the freedom of humans.
We already have precedents for conpanies being forced to impartiality- utilities. Ebay's a utility, the modern large-scale version of the ground a flea market sits on.