Is it really expensive to build a website like Amazon, eBay, Etsy or Airbnb?

It is costly. To build a site in the magnitude of those eCommerce businesses requires a lot of resources, capital, patience and research. You need to study the customers of each site, the type of info each business provides, their legal T&C, etc. Find out the advantages/disadvantages of each site, their unique abilities and how user-friendly they are, etc. Then, you need to acquire the technical resources to create features for admin, such as: dashboard, reports, analytics, invoice & order management, shipping methods, tax management, etc. Features for sellers, such as: seller dashboard, product type, internal chat capabilities, multiple product support, etc. And features for shoppers, such as: social logins, single page checkout, reviews, ratings, multiple currency support, user profile page and more. To all this, of course, you need to consider the cost of front & back-end tech, integrations, platform selection and more. Breaking down the costs: Platform - between $300-$1,000 one-time fee, although there are SaaS options starting at around $30 per month. Design - between roughly $300- $500.  Your store design is crucial in attracting customers, not only in the way it looks but also in the way it operates, from start to check out. Web Hosting - you will need a server or SaaS solution to host your eCommerce store. Typically starts at around $15 a month. All in all, you can expect to invest a good few thousands of US Dollars on an eCommerce website that comes close to the level of Amazon, eBay, Etsy or Airbnb.