Many entrepreneurs put themselves at great risk of facing a catastrophic situation by relying on a single or few sales and marketing channels for the product. For example, if you only sell through eBay you are at their mercy when handling transactions with your customers. What if you have a disagreement with eBay and your account would be suspended or terminated? You have to protect yourself by using multiple channels.
You may also put yourself in danger if your sales consist of specifically one or two customers. Having a strong mix of customers is critical to your long term success and establishing a stronger value to your business. I would be concerned if any one customer or marketing channel controls more than 20% of your business.
Don't get me wrong, I'm never one to turn down business that I am capable of doing, but you have to protect yourself from losing that source and even work harder to build up your customer base and sales channels to balance out that business.
If you are unable to build alternative channels, you know that you have to adjust your business model to anticipate the loss of the source so that you have the resources in place to protect you in the event you do lose that customer or marketing channel.
You can do this by holding back x amount of the sales profits and putting them into a safe investment. Have enough cash reserve on hand to wind down the particular business channel and adjust the business accordingly.
Take your time and plan for your long term success and your business and entrepreneur career will have a healthy long life.