I know this thread probably should not continue here but I do want to help by describing specifically why this might be tough to sell on ebay.
There is a big gift card/certificate category on ebay. I find it a little scary but people buy and sell gift cards and store certificates. The snag for you IMHO is that ebay has rules about listing items in the specific categories that they belong. So, ebay would require you to list this store credit (I don't know that it is even transferable--BalNY may make it out to you only) in the gift card/store credit category. It would be against ebay's rules to list it under handbags because it is not physically a handbag. They have a name for this infraction, but it slips my mind right now.
Sooooo, anyone searching for the keyword Balenciaga on the entire ebay site would find your listing, but anyone searching only within the handbag category for the keyword Balenciaga would not see your listing. Meaning, a lot of people would never see your listing because many are searching within the handbag category.
Before selling the credit on any venue, you might want to check with Bal NY to make sure that the credit is transferable to someone else. With their tight return policy, you may not be able to give the credit to someone else.
BTW, did you get your new courier yet? A blueberry courier sounds so pretty.