They don't want easy money, as I mentioned before. Its useless to "overcamp" things, so doing the "easy money" stuff with 100 people would result in less of a profit then splitting them up. So gilsellers in sky will never really end unless the demand for a any items that drop in sky goes away. Which, quite honestly, it never will... heh
I often draw paralels with other companies in real-life (since gil sellers are a real-life company behind the computers). In this case I'll compare it to cloth stores, since my family owns one. When you restock your storage, you don't concentrate on the things that sell easy for much money. While yes, you buy more of those, you don't ignore everything else. Buying 1000 of the same thing that sells easy will mean you have too much of it in stock, resulting in your not selling everything. Instead, you buy less of it, and also order things that don't sell as well, but still sell at a steady pace. This way you get an income from several different things, thus resulting in a larger profit over-all. Selling gil works the exact same way.