When most of the significant cost is driving the non-paying side to to the site which in turn will attract the supply side?
I'm the CTO of https://3dagogo.com a marketplace of proven to print 3D designs.
We look at the two sides differently. There's not a single customer. In our case you have designers and purchasers ( sometimes the same person can be both ).
Cost and methods for acquiring designers are very different than those to attract purchasers.
I would clearly separate the sides and come up with separate cost structures.
In my opinion when you're looking at the marketplace from the purchaser perspective, the other side's acquisition costs can be seen as fixed marketing costs.