They do by soft power -- leveraging their position in the market to make the alternatives completely unprofitable, thus literally preventing you from selling on other platforms.
the word monopoly gets mistakenly used in place of anti-competitive all the time.
Are you describing predatory pricing? Where they sell at a loss to put competition out of business or to grow faster? Or are you trying to say its not profitable to sell on ebay or shopify instead of amazon? Amazon using its scale to clone products and sell them with low margins, in an effort to prevent competition from springing up is anti-competitive, but its not them being a monopoly.
They do by soft power -- leveraging their position in the market to make the alternatives completely unprofitable, thus literally preventing you from selling on other platforms.