Here are my thoughts on the topic.

I think in order to be successful with display advertising, you have to have A LOT of traffic to your site. Realistically, for most people it just isn't feasible, due to the sheer amount of work involved in getting your site to the point where it can be profitable for you in any kind of substantial way. Although, this is definitely the easiest route to take.

Of the three, I think your best bet for making a substantial amount of money is affiliate marketing. There are an unlimited amount of ways you can be successful in this, but there are people that make a LOT of money doing this (like millions of dollars annually). However there is also a big difference between putting up one affiliate link to someones ebook on your blog, versus spending 16 hours a day doing keyword research, split testing and risking thousands of dollars in cpc or cpm affiliate marketing. If you are serious about making substantial money and are a bit of a risk taker, this would be the way to go. And frankly, even including basic affiliate links on your site to informational products will probably yield better results than advertising in most cases.

Direct sales, in my mind this is in the middle of the other two. If you have some knowledge that you could impart on others in a useful and interesting way, creating an informational product of some sort, is a great route to take. You won't make millions, but there are definitely people out there making some decent income from their products. You also have the benefit of being able to create it and forget it (more or less) to get income for an indefinite period of time.

By using a combination of all three, or at least understanding the basic concepts of all three, you will increase your chances of success by figuring out what works for you!

Hope this was useful!

I would think direct sales – producing something of your own rather than reselling someone else's product – would potentially be the most lucrative. The trick, of course, is creating something that people are willing to pay for and marketing it well. If that part were easy, we'd all be doing it.

Information products have a leg up over physical products in that they're truly a create-once type of thing. Once your ebook is written, you can sell it over and over for only the trouble of maintaining your web site and payment system. Physical books are close, in that your publisher does all that for you.

Physical products that you create have the greatest potential, but they're also a lot more work. Production, distribution, and order fulfillment are all issues here that don't exist with the information product. Much of this can be contracted out, but that only makes financial sense after your business is established and you can afford to put more money in for the sake of efficiency.

I make money by the first two.

Display advertising is the most steady of the three. It's also the easiest to implement if you have good searchable content. Google has it figured out.

Affiliate advertising has a lot more variability, but sometimes it can be a huge windfall if you hit with the right content at the right time. Reviews and explanatory content related to a particular affiliate do far better than just plopping up a banner and waiting for people to click.

Direct sales are the hardest to come by but they can be the most lucrative. Write something once, sell it again and again.