Question: Why is locking the stores considered private (inside) rather than public (outside)? It is obvious to all in the street that the stores are locked.
Locking the stores is considered private, because "public" means that they announce for all to hear the troubles befalling the community and the prayers being recited (such as the prayers that are said in the street of the city). Closing the stores is certainly noticeable, but it does not clearly demonstrate that a Ta'anis has been declared, because there are other reasons why the stores might be closed (as the Gemara says in Berachos 17b regarding people who are not working on a Ta'anis; it is not considered arrogant self-proclaimed righteousness, since it could be that they weren't working for other reasons).
M. Kornfeld