This tutorial shows how the main data is calculated when you list a Grouped Product.
Before listing grouped products read eBay Multi-Variations requirements.
A variation lists and appears on eBay only if its product Status is Enabled and Stock Availability is In Stock
Product ID 2066 is a Parent Product that contains IDs 2065, 2064, 2063.
This product in Listing:
eBay Available QTY is the sum of quantities of all associated products:
50+27+10 = 87;
"Buy It Now" price is the min price of the associated product that can be bought on eBay: $5.20. 
Note
M2E Pro lists Magento Grouped Product as a Single Item on the Channel by default. To change the settings for the List Grouped Product as option, go to eBay Integration > Configuration > Settings > Magento Inventory > Variational Product Settings.
Check out this article for more detailed information.
eBay only displays 30 characters of the 55 characters allowed in the drop-down field for Multiple-Variation listings. Click here to find out how to set appropriate Title for Variations of the Multi-Variation Item.