You're not sure why someone would sell a business just because it's in a big industry? ???

If you start up a business, and let's say get it up to $10,000 per month profit in the gambling niche (a giant industry)... and you're making $10k per month with it, but someone comes and says "I'll take it off your hands for $250k", would you consider it?

If it's your sole focus, your "baby" or you have big plans for it... then maybe you'll hold on to it, build it and later sell it for even more.

But I'm not sure what the size of the potential market has to do with it.

Why does any business owner sell any business? because it frees up their time and gives them an exciting payday. In my opinion, the reason you BUILD a business is to sell it.