Charging our sellers on product listing.

[FerPidg]

Hi all,

Basically me and my team are creating a marketplace where a seller can list a product, we collect it, process it (eg. verify that it all works and is up to par with the listing description) and then make it available on our shopify store. All that said and done, we dont want anyone to just list any product at any price and expect us to collect it and verify it. That is why we have thought about charging the seller, (even if the charge is withheld until either: a) the product is sold or b) we return it to the seller for one reason or another) some form of a listing fee. 
Logically if the product is sold the charge will be refunded minus our margin/comission. 

I have contacted the devs of the marketplace app we are using currently to see if there is something I have missed but I am interested to know any other apps or ideas that could make this process happen.                                                                                                                                                                                                                                                                                                                     (We are using Webkuls Multi Vendor Marketplace atm but we might switch if we find something more suited to our needs)

Im also pretty new to shopify so it is possible I missed some vital information when describing my problem, if so Ill try add it asap.


Thanks in advance

- F