Say there is a marketplace company that facilitates buy/sell of digital content (i.e. marginal cost = 0). And that content is somewhat substitutable but not completely so. As an example, for a particular category of digital goods we see the majority of sellers pricing at $100, while some price lower and higher based on whether their good is clearly much higher quality (or has more inclusions or whatever) or lower if lower quality/less inclusions/more simple.
The marketplace levies a commission on the sellers, say 30% of the sale value.
And now the marketplace decides to increase the fee to 40%.
How would the sellers respond? Do they just soak up the extra fee? Do they pass it through to customers? The marginal cost of additional unit sold is zero. The company is somewhat of a market leader (but by no means the only option for buyers).