system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How can 3rd-party sellers on Amazon sell their products cheaper than their standard retail price?

I'm talking about how some sneakers are cheaper off Amazon than if you go to the physical stores or how Ralph Lauren polos are cheaper on Amazon than in Macy's. Do the sellers get the products straight from the source or are these counterfeit? And if they are, why isn't Amazon doing anything about it.
gpt
Places like Macy's or JC Penny have a set profit they are trying to achieve with the products they sell. They buy shirts from Ralph Lauren for $10 (probably a lot less than this) and sell them for $25, making sure they get maximum profit to pay their employees, brick and mortar locations, etc. 

Places on Amazon don't have as many expenses and usually don't have a set profit margin they want to make on every product. This means they can charge whatever they want for the product. 

Amazon doesn't do anything about it because they get a set % of what these sellers are getting, and these sellers are doing well because they are undercutting the competition.