Let's say you came up with the idea of something really unique and there are no competitors on the market. In that case, the pricing decision is simple because you're free to set any fee (even quite high) you find appropriate.
But more likely there are already several or numerous channels your potential sellers already use. That's why your offer must be competitive.
The online marketplace center Etsy was started in 2005 and it had to compete with eBay and Amazon. So their pricing strategy was based on setting much lower and attractive fees to sway sellers from major marketplace shopping cart platforms
If you are trying to beat a number of strong rival virtual multi-store marketplaces you have to offer something they don't have, whether it's a smaller commision or greater value.
Do you know why photo stock services like Getty or Shutterstock are able to charge up to 90% from a photographer? Because they are the biggest stock services offering an impressive selection of images thanks to the number of vendors. The wider selection they offer the more value it has for a customer. Thus if a photographer wants to sell his works the chances are much better with big photo stocks.
This scheme may not work for shops where the services are identical, for example, transfer services. If existing providers are enough to meet customers' needs there's no point in increasing the number of providers.
Thus the more value a product selection has for a customer the higher fees the owner could set.