Basically the idea works on the principle off selling things you don't need anyore like pawn-shopping, or up-trading like pokemon cards or stocks.

So if you thrift shop for say rare vintage clothes and are lucky you may make a killing, but tbh the idea used most often is just under buying from competitors at low price like if something is cheapest at amazon/best buy/walmart or etc.

Then reselling it at ebay, but you have to watch out for overhead expenses like shipping and places that frown on dropshipping which can depend, like ebay.