Instead of the numerous small purchases for turquoise, why not create a premium account for a reasonable fee that gives enough value that people would want to purchase it? Some examples of what could be offered:
-extra barbarian lootings every day
-some reduction in time or costs for buildings, troops, and/or research
-a protection bonus if you are attacked (fewer troops lost)
-one-click resource collection for all cities
I'm sure there are many more.
In a game I played previously, I always went for the premium account because it gave so many advantages for a reasonable cost (about $1/day). Virtually anyone can afford that. It's way less than the cost of a cup of coffee or a couple of cigarettes, and people always manage to find the money for those. But if they can't, make it possible to donate a premium account for a period of time to another player, or to donate turquoise from your own account to someone else.