Hello.
First of all, I want to say I have wide experience on how alibaba works. In the past I've earned some money by buying alibaba/aliexpress products and selling them in some marketplaces (eBay and similar) and self-made websites.
Now, I have a good idea about how it works. However, I would like to talk about functionality. For example, when a seller uploads a product, it will need to be reviewed before it goes online? Do you want your web application to have an import tool so the high volume sellers will be able to batch upload their products? Having in mind that we are talking about a B2B website, will buyers have the option to export products from the website so they can import in their respective platforms? Do you want the design of the front/back end to be responsive?
All these things can make the quote to change, both in price/time.
Interesting point: In the past I've developed an intra-net web application made to manage product profiles/categories/products themselves. In order you to have a quick idea, profiles were linked to categories and categories were linked to products, so for example, the profile "smartphone" will ask you to input all features a smartphone may have (cpu, ram, screen size, touch type, etc.), so if the category "Smartphones > Samsung" is linked to "smartphone" profile, everytime you intend to upload a product in that category, the application will ask you the adequate fields to be filled.
I hope you will feel interested in my bid. ;)