We use Brightpearl for our multichannel business with web sales www.katiekerr.co.uk & two shops. It is our central system for managing stock, accounting and customer service. It integrates with Magento for our web store, has its own point of sale system integrates with Mailchimp and when we eventually get around to selling on Amazon and eBay it we can use it to manage our stock for those channels too.
Like every system its not absolutely perfect but its being constantly developed with new features being added all of the time. Well worth looking at.