All of the answers to your question seem to be missing the point. He is talking about buying a used or discounted item and trying to sell it on for a little bit more. The point is to experiment with your sales technique to learn what works and what doesn't, and also simply to practice selling.

Why might someone pay you more? It is as simple as presenting the product in a better way; in a way which gives the buyer more confidence or otherwise makes them more likely to buy.