How does reselling make money?

Could you elaborate on the mechanics of making money through reselling? I'm curious about the specific steps involved in identifying profitable products, acquiring them at a lower cost, and then selling them for a higher price. Additionally, I'd like to know how resellers can ensure that they are making a sustainable profit while still appealing to buyers and staying competitive in the market. Is there a particular strategy or set of skills that are essential for success in reselling?