This is probably the question we get most often. There are a few ways to answer this.
The first reason this isn't an issue is because of the top level view of what these codes allow you to do. Remember that buying code, re-skinning, and publishing is probably not going to make you a million dollar app. But it has a good chance at being ROI positive over the long term, especially relative to building your own app.
So, instead of thinking about the crowded market (which only really matters for you long term), think about how you can use this to make more money than you spent, then repeat. It cuts your costs and increases your chances of success because it's been proven already.
Secondly, it's easy to look around and think that everyone is using codes, maybe even these codes in particular, and flooding the market. I can assure you that the people using this strategy are a small percentage of the overall app developers in the world. With tens of thousands of apps being uploaded every day, the people using the same code are not competing with each other, they are competing with that entire population.
Thus, using these codes actually isn't competing with other source code buyers, rather it's a way to get ahead of the other 80% of developers who are trying to do it the old fashioned way.
That's why we build our own codes - so that the people who find us have an advantage!