Unfortunately, once you have convinced yourself of your own lie, it is very difficult for you to extricate yourself from it, because this means admitting to yourself (and everybody else around you) that you were wrong, and most of us just have way too much pride to do so.
Sometimes, the "sin" is merely promotion of "woo", such as those "magnetic bracelets" or some "super-duper-hyper juice" that is somehow good for everything under the sun (and moon, and stars). If you believe somehow that it works, and you bought it, then you will cheerfully say things like "ever since taking / using _____ I feel much more energized! Thanks! I would recommend this to anybody!" without considering whether you really really believe this stuff works, or simply because you WANT to believe it works.
It's one thing for you to use it, as the only thing it hurts is your wallet. It's another thing completely to actively sell it to others, and trying to convince them, the way you convinced yourself, that it works great.
And it's yet ANOTHER thing to convince them so you can pocket profit, either from they BUYING it, or further, from them SELLING it to other people.
All just because you *want* to believe it works, but you don't really have proof that it works. Later on, you *have* to believe it works, because you can't sell a product you don't believe in. You are now both emotionally AND financially involved. So, are you really selling the product, or your dream?