Keeping track of Items through Platform Notifications
*Developer using company username because I don't have a personal eBay account.* I'm tasked to create a server application to pretty much keep a copy of our items on a company controlled server so we don't have to do costly transactions against eBay and easily update our items by doing our edits and submitting it against eBay's File Exchange, especially since the policy change about contact information. And so far I have server code written up in Node.JS albeit comparable to a dumpster fire, but hey eBay's sending platform notifications to it and the server is acknowledging. So far I've supported notifications that have the 'GetItemResponse' (ItemRevised, ItemListed, etc) element in it, but I'm noticing items tend to slowly veer from their actual quantity, so at this point I'm feeding big .csv files from the File Exchange to get the items to update on the next pass when there's such discrepancy. With a bit of poking around I managed to figure that I might need to support the 'FixedPriceTransaction' or more specifically any notification that has the 'GetItemTransactions' element, but I'm coming here to ask if I'm right to add support for that kind of notification if my aim is to keep items up-to-date? Can I simply take Item.Quantity and put it into my database as the amount we have available? Or do I have to do Item.Quantity minus Item.SellingStatus.QuantitySold if I'm understanding the language?