Best way to add/edit a listing

[eluser]CIfan1000[/eluser]
Hi,

I've been trying to figure out how to do this for a few days, and any help would be much appreicated:

I would like to have a single controller that can add or edit a user's listing. A listing in this case is something the user has for sale. A user can have multiple listings.

I thought of passing parameters in segments to the controller - including 2 segments that would pass as 2 parameters to the controller's index function. eg:

to edit a listing: controller/index/edit/listingID
to add a listing: controller/index/add/null

That worked great until I submitted the form - the action/destination of the form is the name of the controller, and does not have the parameters in it, so I get an error message saying the parameters for the index function are missing.

Then I thought of having an edit function and an add function in the controller, but then I don't know how to do the validation and loading of the view in a single function, such as the index function.

I would be grateful to hear from anyone who has done this.

Thanks!