-How controlled were prices in different kingdoms across Europe?
-What sorts of goods were controlled?
-Were prices fixed, or was there a price ceiling/floor?
-I seem to remember reading that prices varied in different counties in England (a shilling was the cost of a sheep in Kent or a cow in Wales, something like that). Who set all these different prices?
-What was the primary goal in controlling the market this way?
Price controls were generally in the purview of individual guilds. The way they operated varied vastly from place to place, and their influence and industries they controlled depended on the size of the city they were in. There was no one universal system of price control, nor were price controls the same everywhere.
I wrote about the development of the guild system here although there's definitely more room to expand on the nature of pre-modern economic policy.