What is better ?

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When I started flipping sneakers and cards for extra cash, I made one mistake that cost me way more than money: I kept ALL my profit instead of reinvesting it.

Here's the math nobody tells you: if you flip a $30 item for $60, you made $30 profit. Spend that $30 on pizza and you're back to zero next week. Reinvest that $30 into two more items and you've just doubled your buying power. Do that for 8 weeks straight and you go from one $30 flip to sourcing $500+ worth of inventory a month — without ever putting in more of your own cash after the first purchase.

The other thing that took me too long to figure out: track your days-to-sell, not just your profit margin. An item that nets you $10 profit in 2 days beats one that nets $30 in 3 weeks — because speed is what lets you compound your capital.

If you're a student trying to build a real side income (not a get-rich-quick scheme), start with ONE category — sneakers or trading cards — validate it works, then scale. Happy to answer sourcing questions in the comments.