Steve,
We have an RMS/ecommerce link (some of our sites are
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and
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Whilst its not HQ friendly at the moment, because of the way its architected it would be simple to show a combined inventory total on the web. e.g. if 'store 1' has 8 footballs and 'store 2' has 1 football the website would show 9. The link we've created also has full support for matrix items, which some links don't (see size/colour variations on
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and
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Our biggest change would be brokering out the resultant orders to the weborder functionality in StoreOps. This might not be an issue as you don't mention sales order processing in your original post. I can see a way to make this work, but it would require further development (and thus cost).
Is this the sort of solution you are looking for?