Sounds like you are confused about what AIR is. Essentially, AIR is a desktop run-time environment for Flash/Flex applications. This is different from the standalone Flash Player in that AIR gives you the power to look, behave, and interact with the system (to some degree) like an independent desktop applicaiton.
As it pertains to Flex development, AIR is a build option and carries with it a library of classes specific to the target environment.
To answer your question, the effort required to move in to an AIR app is pretty minimal.
providing greater details
1.seller will design a template by dragging/dropping controls(like -tree,button)
2.seller will save it when they finish it.
3.then they will give this template to buyer.
4.the thing is buyer will view this template(actually install it first) in their machine as AIR.machine can hv internet or not.
5.when the seller changes the templates and save ,the AIR will update itself automatically.(if machine has internet connection)