What's the best app for managing inventory across multiple channels?

[CyclingLove]

Here's what I am looking for.  

I suspect I need a third-party app for this.  I sell clothing so I have multiple SKUs too.  I am running 4 different channels currently.  Shopify, Ebay (and multiple international Ebay sites), Facebook and a dabble of Amazon.  I want something that's going to maintain the correct inventory for those.  

Right now I am currently using the native Facebook and EBay apps.  

My products on Ebay are sold at a discount and so I list the items at retail but then use Promotions Manager to mark the price down.  

With the connection between Ebay and Shopify, it will work fine for a while but then suddenly the promotion price is gone and it will go back to full price.   Additionally, my inventory on Ebay will also be erased completely.  Characteristically, Ebay people seem to care less about supporting their crappy products.  

I don't want the app to exchange prices (especially if I switch on Amazon products). I only want the inventory to be deducted and nothing with prices.  

I would be considered to be a fairly light user, selling under 20K a month.  

Let me know which app would be the best fit for my needs.

Doug

Cycling Love