We're motivated to buy things for different reasons. Your job here is to find out what motivates people to buy your product. It can be different things, but you need to pick the most important motivator in order to make a compelling case for your product.
Kids buy a football to have fun. You might buy new running shoes to stay healthy. I buy books to gain knowledge and others play Clash of Clans for entertainment.
A unique selling proposition can be anything, but it’s up to you to find out what it is and take advantage of it. Oreo is notorious for its twist, lick and dunk campaigns showing consumers how they can eat the sandwich cookie and M&M came up with the “Melts in your mouth, not your hand” tagline. So, what’s your USP?