configure and start e-commerce open source

I need to start a multi-shop, multilanguage e-commerce website using a proven open source software. Any shop will be managed by a single vendor, who has the total control of their items, prices and contents. All the items for sales may be exposed on the Mall, from where the buyers may buy items from many shops, may browse all the items and may place a single order. The software should parse the main order generated by the customer to generate orders to the vendors. All payments and sales must be executed, traced and reported (Paypal and credit card). The first step for the project will consist just to download , configure and test the software with real transactions. The test will consist in the simulation of sales and production of reports using a limited number of articles. This will complete the job. Next, I will continue to test the website with the purpose to write the specification for the second step of the project (i.e.: new reports, export/import data from existing database, tuning of the configuration, new features). Finally, I will propose to start the second step of the project according to a detailed specification document. The third step for the project will include the addition of new features and modules. The budget for the project is referred to the step 1 only. next steps will be quoted according to the new specification document.