When I say 'template' I suppose I mean, sell a website you have already designed, so that we can modify it to suit our own customers. We would not use/need the existing text or images, just the layout with with navigation. Your original customer should have no concerns if the design is used for another type of business or even in a different country. Of course, they could legitimately complain if a competing business in the same district used the same outline.
It is no different for those of us who use Xara templates, often using the same design for multiple businesses. And if your original customer does not have a proviso that you, the designer, cannot use the design elsewhere, or sell it to another web-designer, then there is no redress there either?
This would add a value-added service to your business and it would be essentially free to you, as you have already designed the site AND been paid for it already.
Just sayin....