The best financial advice I've ever received is: don't spend money on things that aren't worth anything and don't work on anything that doesn't yield results. Simple advice to understand, extremely difficult to put into practice.
Is it worth it? Is this product worth your money? Is this thing worth your time?
It seems like this also might be helpful advice when it comes to selling stuff. Maybe people aren't buying what you're selling because they don't think it's worth their time or money? They probably don't care enough, or trust you enough. For one of those two reasons, it's not worth it.