Arbitrage is a gleeful scenario when you find someone willing to pay you more for a good than what the free market charges for it. If you're paying $234 per ton of copper ore concentrate and I've got a seller who can get it to me for $220, I can make $14 without risk.
Magic cards behave like this, too. Though you'd think mass-market sites like TCGPlayer would make store buylists coordinate with what the market is charging, sometimes that's not the case. The result is a card can have a negative spread. You can buy cards online and make a profit selling them to buylists.
You can manually do this, but Trader Tools has the ability built in. Check out this six-minute video to see how you can use Trader Tools to spot arbitrage opportunities.