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The biggest concern is duplicate content. If the domain acmewidget.com points to widgetzone.com/acme-widgets and that directory is accessible from the widgetzone.com website then the search engines will see acmewidget.com as a duplication of widgetzone.com/acme-widgets and almost certainly exclude it from the search engines. I'd recommend setting up microsites under your keyword rich domains that sell a small range of products - the work load is manageable and as long as you provide some unique descriptions and content will work alongside your main website.