What is the value proposition you can build on top of this? create marketplaces but for what? is it common to use a business model to skim percentage for each transaction?
Yes it is a common business model, and great work if you can get it.
To be successful at it you need to be more than just a middle man; you provide value by solving an impedance mismatch between sets of buyers and sellers.
For example: eBay provides a reputation system, a search engine, and a payments processor (PayPal). Those are all arguably difficult things for individual buyers and sellers to provide. As a result eBay attracts a large audience of both, making the marketplace for goods more efficient, and extracts a small cut of lots of transactions.
Stripe is providing a way to make the payment provider part of the marketplace equation more easily available to developers. In theory you can focus more on the other hard parts of making a successful market.
This is for products like TaskRabbit, Lyft, Postmates, HomeJoy, OrderAhead, and so on -- products that happen to involve some amount of each charge going to a third party. They generally aggregate the demand and build the UI themselves, though -- without them, there is no product.
[Edit: yep, what Zac said in the sibling comment.]
It's a pretty common model -- create some valuable way for buyers and sellers of some service to connect. That's AirBnb, Uber, Lyft, Ebay, Postmates, etc. It can be a hard business to start because of the chicken and egg problem but often leads to a very defensible business because of the network effects.
