In reply to Doug MacKenzie, I do not think it is the case that Coase thought that transactions costs were always high, although he certainly thought that they frequently were.  We know of cases where they are not and where with property rights well defined we observe market solutions that internalize would-be externalities.  A classic and much cited example is that of the beekeepers and orchard owners, written about by Cheung.

Barkley Rosser