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Good Til Canceled (GTC)...
are necessary so your listings don't end when the quantity reaches 0.
GTC reset each month on eBay and a new insertion fee is billed by them.
If you want to create auction listings or learn more about them, click here.
You save money with GTC because a normal "auction" the listing ends after it sells. You have to create the listing again if you want to sell more and eBay charges for this. If your listing is GTC, you keep the listing live and you can sell more with no insertion fee from eBay.
An auction listing has a remaining time until it ends
Here's an example of an auction with time remaining...