Go through any thread started on GFY by a noob asking for the best way to get started in this business. In every one of those threads there will be a couple of people suggesting the noob start with a niche he personally likes. Why? because you need to understand your target audience and what it likes.
There are a couple of niches that I personally understand pretty well. I know what converts, which sites convert, which sites retain, which sites convert US traffic, which sites convert EU traffic, which sites don't convert EU traffic, which sites update,... I know and recognize old content that has been circulating in those niches for a long time, I recognize fresh content. I recognize sites with nonexclusive filler content and I recognize new content. I recognize tours that will work with certain types of traffic and I recognize tours that probably won't work... Because I know all these things, I'm able to convert traffic in these niches. I have my own sites to generate this traffic and I also buy traffic in these niches. Yes even the so called worthless tube traffic.
Now, I also have sites in lots of other niches. All my sites make money, but these sites have ratios that are way worse than those of my sites in my main niches. Why? because i don't take the time to constantly monitor, adjust and improve my campaigns. Some of those sites will have banners linking to a paysite that hasn't been updated in 2 years, so retention will be lower. Some of those sites will not have the latest/newest/freshest FHGs with the latest *must see content in that niche* on them. Could I improve those ratios? Yes, but there's only 24hrs in a day so that isn't really an option. I doubt any one can know and understand every (micro)niche out there.
I focus on the things I'm good at and the things I enjoy doing. I generate traffic, I rape Google, I focus on the few niches I personally like and I do pretty good in those niches. My sites in those other niches still make money, but to increase their earnings I simply sell part of their traffic instead of investing more time into them. Time I use to do what I'm good at.
So yes I sell traffic. and yes, i buy traffic. I buy about 5 times the amount of traffic I sell. Why? because it's good business...
hmmm, looks like I just explained the basic economic principle of
opportunity cost 