The market cannot be mined because of a typo in the code;
in prototypes/buildings.lua line 29
minable = {mining_time = 1, result = "rpgitems-market"}
should be:
market.minable = {mining_time = 1, result = "rpgitems-market"}

First of all, you are right, thanks.

Second of all, by default it was meant to be unmineable by original author.
However, I'll change it and we'll see how it'll go.

Thanks for reacting so fast, I thought it was a typo because otherwise the line would not have been here at all or as a comment, maybe make it an option if peoples complain about it.