It's risky to rely on one customer for the majority of your revenue.
90% of PriceZombie's revenue was dependent on affiliate revenue, and thus Amazon. Similarly, virtually all of TutorSpree's revenue was dependent on SEO, and thus on Google, and when Google changed their algorithm, they saw a similar fate.
[1] http://www.aaronkharris.com/when-seo-fails-single-channel-de...
It surprised me how little fightback there was to this change. So, anyway, never rely on platforms like that, always assume it's a temporary arrangement (especially if the platform is run by a major player).
Think about all the successful companies out there that DO depend on third parties:
- Buffer (Twitter has a bad day? Cut their API access off!)
- Candy Crush (Apple hates them? Remove their apps!)
- RetailMeNot (Google thinks they're spam! Delist them!)
- DuckDuckGo (Bing hates them? Cut off their API access)
- Heroku (Amazon has a bad day? Cut off their AWS Access)
- MailPlane (Google doesn't want Gmail apps?! Cut them off!)