There are a few advantages to taking this approach. First, you get close to 100% of your listed price (minus the transaction fee of your preferred payment gateway). Second, you are not competing with other authors and have the reader’s attention solely on your product. Third, selling your eBook on your own platform is a great opportunity to build a long term relationship with your readers via email. Fourth, you can bundle your eBook with other goodies in order to bump up the value and make it more unique. With so many advantages, it's worth putting up the time and start building your own platform.
* I use Personal Capital to track all my finances in one place. It’s much easier to use their free software to follow 28 accounts on one platform than to log into various accounts to check my balances. They’ve also got great tools for x-raying your portfolio for excessive fees, recommending a more optimized asset allocation, and planning for retirement with their Retirement Planner.