ShipScript has been an eBay Community volunteer since 2003, specializing in HTML, CSS, Scripts, Photos, Active Content, Technical Solutions, and online Seller Tools.
I was working through the API and found that eBay will store both the listing's original quantity (40) and the quantity sold (12), but not the remaining balance. The listing itself will compute and display the difference between the two (28) as the available quantity (or the seller might have chosen to display a threshold like "more than 10 available"). Nonetheless, even with a vague threshold display, the buyer can simply type different quantities to discover the available stock because an error displays if the number is higher than available stock (28).
When revising the listing using eBay's SYI listing form, the computed quantity available (28) is shown in the editing slot, not the original quantity (40). Editing the SYI number (from 28 to 38) adds the difference to eBay's stored database quantity, which increments the original number (from 40 to 50).
The listing then displays the revised available quantity (38) and the API returns 50 as the quantity (instead of 40), with 12 sold.
I thought it might be important to understand how these values are stored because some sellers have complained that relisting a partially depleted item resulted in the original quantity appearing instead of the remaining balance. Since eBay does not store the balance (this is calculated instead), we can see how could happen.
I suspect File Exchange would adjust the numbers in the same way as SYI, since the "revise price and quantity" report from File Exchange displayed the same computed remaining balance (28) that was shown on SYI.
