Event souring investigation
This is a mickey mouse sample just copies content from the following

https://github.com/gregoryyoung/m-r
http://beingtheworst.com/
https://github.com/beingtheworst/btw-samples
Lokad
http://msdn.microsoft.com/en-us/library/jj554200.aspx
http://channel9.msdn.com/Events/aspConf/aspConf/CQRS-with-ASP-NET-MVC-A-Year-On

Need to run a server see test\esinv.tests\readme.txt

Too many abstractions
Skip repository and use event store directly ?
Merchant repository should be based on the event store also
Should i allow multiple key types ? Guid for order, int for merchant
How do i get the merchant from the event store unless the command includes the merchant Id
How\Where do i publish events so they can be subscribed to externally ? Bus will need to cater for internal and external events
Throwing exceptions on command failures - not recording attempts ? .i.e Commands are expected to succeed when they get to the handler

Payments must all be with the same currency
Payments can exceed the order creation value
Refunds must all be with the same currency as the initial payment