I think this is a quick clue to marketability: how quickly can you explain what your business does? Can you do it in 15 seconds? Can you do it in 25 words or less?
Two- and three-word explanations generally mean marketing is relatively easy: gourmet restaurant. Limo service. Web-based to-do list. Online project planning. Gardening blog.
Longer explanations can indicate problems with marketing. I can’t give easy examples here because the business offerings that take longer to explain are harder to remember.
I’ve had two examples that are memorable failures for me, in my business, that I link back to the problem of not being instantly explainable. One was an Excel add-on called Forecaster, which allowed the user to do spreadsheet projections by drawing a line with the mouse. Colossal failure. And the other one would take too long to explain.
There are always exceptions. But this explainability factor is a good general rule. Try it with your business, or business idea: how long does it really take to get the idea across?