Advise on Multivendor Ecommerce Platform

Dear Freelancers, I am planning to open up my mult vendor platform which can have unllimited vendors and unlimited buyers account. Looking to build something like Amazon or Flipkart. The platform should be scalable and secure. The technology should be capable of handling the future technology upgrades and vast volume transactions. Also would like to know the difference between having templete and custom designed code. Please do share your techincal advice on my start up. Appreciate your help. Thanks.