Need advice on a dev project.. Creating Ebay style peer to peer shipping..

I am currently having a website built for me that consists of an online auction (ebay style) variety.. I wanted to build into it a shipping and label creation to go with the site..

The easy part was finding a shipping api that users could use to ship items.. but the problems of setting up such a thing are incredibly difficult. Such as figuring out a way to get the labels payed for if the site was to pay for the labels using the easypost api we are using.. So the way we have it setup is a customer would go to ship an item by getting label set up.. then paying me for the labels creation and whala! Label..
Heres what I want to know.. Does anybody or can anybody think of an easier way? As it stands customers would have to pay the fees associated with the transaction making label creation more expensive. Refunding the labels the same.
Also customer to customer transaction problems are also difficult.. Such as if a customer has an issue with not receiving an item how as the online auction site can it force a refund of that item with shipping. Has anybody heard of paypal having an api that allows customers to set up something similar to what ebay has?
If anybody knows alittle something about this, I am ALL ears!