How to introduce users to a new buying process?

We created a B2B marketplace to let merchants and retailers trade their goods in a particular market. The buying process is similar to eBay: you can bid to an offer or instead buy specific amounts for a fixed price. The problem is that the merchant who created the offer has to accept it, so the user needs specific feedback when creating a proposal to buy something.

What is the best way to explain or give feedback on this process, so the user will not be confused when actually buying something while still having to wait for the confirmation?