I personally used Kagi and eSellerate long ago. But I don't do that anymore, because I'm in Europe. They do all processing in US Dollar which means for people like me that all money from Europe is converted to USD and later converted back. This conversion costs a few percent every time, so having the account working completely in Euro is an advantage for me. And that is done by Share-It.
For Stripe and Fastspring, they are fairly new and maybe more flexible to add an option you ask them for. Stripe is currently US only anyway. For Fastspring we had a nice meeting with one of their managers in Atlanta on one of the evenings after the Real Studio Summit earlier this year.
On the serial number creation, several services have APIs to hook into. For Mac App Store you don't need serials. For Paypal, you'd have to deliver yourself as it's only a payment service. For Share-It I once made a template for a serial number generation application. You can create this app in Real Studio and add your own unique serial number generation code. Than compile the app and upload it to Share-It. As serial numbers are now generated on the fly for payments, you can simply relax and wait for cash and support calls.
If you need help creating serial numbers, we have our RegistrationEngineMBS class. And if you need dongle protection, you can use our dongle plugin. But that's more for expensive products with a few customers.
What experience do you have?