We have a major problem now with using M2E in that the license key is based on the IP address and we now use a hosting environment where our IP address changes frequently. Because of this, our license goes invalid and we don't catch it because we get no notifications and we unfortunately don't find out until we over-sell.
The next problem is that once we validate our license again, the inventory is incorrect due to all the sales during the time it is inactive, even in the M2E program itself, which I don't understand as it should be the same as our Magento inventory. When I click on the "variations" in M2E listing admin to see the available inventory of a M2E configurable product that is live on ebay, it shows the inventory currently listed on ebay instead of the actual available inventory in magento, and there is absolutely no way I can find anywhere to update that inventory to the correct current magento amount. It is not simply a matter of manually revising the product listing to ebay, because the inventory shown in M2E itself is wrong so it just pushes the same bad inventory again. PLEASE help me figure out how to get the Magento and M2E inventories to sync! I saw that the default action is when an item sells on magento or ebay, it resets to the magento, but that doesn't work when the product is sold-out on magento and there are no more sales there to correct the data and so the only option is to over-sell on ebay to get a "sale" so it resets to the correct sold-out quantity from magento. That is a very, very bad solution.