A common thread among a large percentage of successful entrepreneurs is that they have control over at least one of their own products or services.
Following are some significant advantages of being the prime source:
You get all of the profit from sales of your own products, compared to generally only about 40-50% from other people's.
If you deliver solid value, giving your customers their money's worth (and then some) many will want more of what you offer, putting you in a better position to develop repeat business.
Control over your own products and services gives you control over prices, supply, policies, and so on. The less you have to depend on other suppliers (who may not always be as ethical, reliable or efficient as yourself), the less related problems you'll have.
If your product is unique, and especially if you have exclusive rights to it, you have your own little "monopoly." There is no direct competition with others selling the same thing, unless, of course you set up a dealer or associates program. People must then still go through you - directly or indirectly (through one of your dealers or affiliates) - to obtain it. Either way, you profit.
Now, I'm not condemming the reselling the goods and services of other prime sources, but I highly recommend that you make becoming the prime source for at least one of your own proprietary products or services one of your primary goals, if you haven't achieved that already. And if you've already achieved it, why not add more to your line?
An ideal situation would be to have a line of several of your own products and/or services. Realistically, however, it usually takes time to research and develop (or otherwise acquire) them. Some types of products or services can be developed in a relatively short time, others may take several months.
Although the barrier to entry may be a bit higher than exclusively focusing on marketing the goods and services of other prime sources, once you've conquered it the potential rewards can be much greater.