A retailer should add a router to their equipment if they are looking to produce larger items and for maximum production of smaller pieces.
A rotary engraving machine would be a better purchase if the retailer is doing smaller pieces, detailed work, and custom one-off items. An engraving machine is much better for surface engraving applications (diamond-drag and burnishing) as most engravers have a floating spindle or can be equipped with a burnishing attachment. This allows for the tool to keep a consistent pressure (and line width) even if the material is not perfectly flat.
-Chuck Donaldson, Antares