Repeated in my own words in case I misunderstood: Your middle-school-age nephew is buying used products at thrift stores in other cities with his dad's transportation and consigning the products to his dad to flip on eBay, and that's how he affords to buy legit music.
If I understood your post correctly, that's little different from his dad giving him an allowance, as his dad is providing free transportation and free use of his eBay seller account. Both the transportation and the eBay seller account require being 18 or older. The only labor the nephew contributes is deciding which products are worth flipping.
How does the nephew determine what to buy to flip? I ask because I'm trying to evaluate how well this sort of business would work for my cousins.