Depends if the Amazon->Provider model is more like Yell->Advertiser or Uber->Driver.

If Amazon provides leads but the service provider still does marketing, customer service, accounting, payments, sets prices, insures against fraud, closes the sales and so on, you'd expect Amazon's margin to be pretty slim as they're basically a telephone directory.

But if Amazon manages the customer relationship, does all the advertising and customer service, sets the prices, makes the final sale and all that, so the service provider just turns up to do the job, they could probably make a much better margin.

Good point - that could definitely be worth 20% when done properly. But I can't help think of eBay which does all that and still feels like a hella rip-off. My last eBay run had some international sales and when I did the final sums, taking into account FX charges and PayPal fees, the eBay machine sucked up near 40% of the final sale price.