Arbitrage
Buying from one platform or market where prices are low and reselling on another where prices are higher. Retail arbitrage (buying from Kmart clearance, reselling on eBay) is a common variant.
Where you'll see this
This term is commonly used on:
ebaydepopfacebook-marketplace
Put this into practice
Shopfront handles the hard parts of crosslisting and inventory sync so you can focus on finding and selling.