In the early days of the Internet, web pages were made in many different ways. Some websites only worked in Internet Explorer while others required a slow-loading Java client to view the site.
Not too long ago, the Worldwide Web Consortium (W3C) and Internet Corporation For Assigned Names and Numbers (ICANN) agreed on a set of standards that, if properly implemented, would lead to a better, faster, more secure Internet. Designing a website to standards, means that pages will load quicker, can be viewed on a wide variety of browsers (including mobile devices), offer greater security, and are more easily indexed by search engines.

Sites built to web standards are also scale-able, upgrade-able, and thus cost-effective. Your online marketplace can grow as you do.