I rarely sell buildings in my founded cities. Since you asked, it triggers a thought: My capital usually gets the majority of great people from scientist/engineer/merchant pool, but just about every other riverside/lakeside city gets one, and only one, during the course of the game. If it's a food-rich city, I'll often plan ahead and get the garden, often as something to cue before education, and as the capital, with garden, NE, and all other bonuses spits out scientist after scientist resetting to 0, the other cities will build and build towards the ever-increasing limit, and will trigger once, and fortunately it's always a scientist, and almost a bonus one. After that city triggers, I should really sell the garden as there's no possibility it will trigger again.