The economy and life fluctuates. Sometimes we're riding high with excess spending cash and sometimes we can't afford two shillings. If you keep a customer through the hard times you'll have a customer when they're at their best and if they value that relationship they'll reward you for it. And as everyone hits their high at random times, if your product has real value, you'll always have someone rewarding you
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And V4V gets you something more also. If someone doesn't have the money to pay, maybe they have skills and time. They can assist with making your product better in exchange for more of it. They can assist with marketing, artwork, etc that you otherwise wouldn't have gotten by trying to hire someone outright and having to over pay yourself.