step by step guide on how to use cobalt8 to create a multivendor ecommerce site for a beginner

Wow! This can take half of week to write it in details for even beginner.

In short all you need is.

Create type products

Allow personal categories for users

Create section products

And you will need something to make inquiries. Than can be either email field that will send emails to suppliers or it may be built-in personal messaging system if you install Jomsocial or easysocial. I believe you will need one of those for user/company profiles.