I don't use the inventory option, so I can't advise in that regard, but if it helps, drafts and templates are saved until deleted. To create a draft of an exitsing or ended listing, all you have to do is click 'relist' or 'sell similar', then 'save for later'.

You used to be able to very easily save a listing as a template when editing, but I can't find how to do that now, so the only way I can see to do that is starting a new listing from scratch. 

You may need to re-upload images come September, though.