Amazon charges a referral fee on all 3rd party transactions. It varies by category, but is typically 8-15%. All retailers take similar margins. For instance, the largest retailer in the US is Walmart. Here is a list of their 3rd party referral fees: https://marketplace.walmart.com/referral-fees/.
Given that the retailers all take similar margins, I think it's crazy to somehow paint this as Amazon forcing a markup on customers. All Amazon is doing is refusing to show products if the price on Amazon is higher than at a competitor. It's the seller who chooses what to do about that. They can either raise the price at the competitor, or they can lower the price on Amazon.
Again, the retailer's margins are similar, so it shouldn't matter.