If you want your sellers to buy from you try selling them what they need, if they already have other sellers then make yours more attractive.
Let's assume you want to become an online seller on a peer-to-peer Bitcoin Marketplace, and one of your main concerns is "who will my stuff" or "can I sell fast enough", then you have an idea— you decide that instead of buying things with fiat currency from local sellers who hardly consider buying from you, you will be better off converting that fiat currency to Bitcoin to buy from local Bitcoiners who will also buy things you sell. You look for sellers on the marketplace who sell things you frequently or occasionally buy, and then ask them the kind of things they frequently or occasionally buy in order to supply them... But under the condition that you would mostly buy from them if their products/services are top notch (good quality) and more competitive or attractive than your former sellers, and you likewise promise thesame thing. And then you start buying from these peers and they start buying from you after comparing prices and quality of your stuff with their former sellers'
Your take?
In a case when the dealer or your supplier is not in need of your products and you need theirs to buy and stock your store according to your customers demand what will you do? I trust you can not give them the conditions that they must buy from you before you buys from them and you can not either ask them to diversify into another product of your need conditionally.
Except it has been reviewed by their market team and management and find out how profitable they would be if they venture into it because I am sure they can not afford to loose while you profits.