Try reading the ebay policies, terms and conditions you agreed to when you opened your store. One of those rules is that you should not list anything that is available elsewhere.

As items being out of stock is one of the things that really annoys buyers ebay have quite rightly decided that being out of stock is a very good reason to give a seller a defect, each disappointed buyer is one who may never come back to ebay screwing things up for the rest of us who abide by the rules.

I doubt if you will get any sympathy from either buyers or sellers except of course others like yourself who think the rules don't apply to them..