I know there is a page on this exact thing, but in this case there isn't any error or message at all for anything. The shipping address is accepted, the payment method is accepted, the items I'm ordering are in order (although strangely, there are no item numbers anywhere for any of them), it accepted the gift card code I put in and the amount has registered, the Complete Purchase button is still greyed out (with no error or suggestions) with a red circle/slash where my cursor is. I've tried ordering them one at a time and with only one gift card option, but still nothing. I've cleared cache and checked the individual items and they're all in order and still there at the time of writing this.
I noticed this happened after some maintenance was done. Everything from my browser to my settings in everything are all still the same as when I ordered from Ebay before, so it's not anything I'm using. So far, no agent has come up with a solution, but they did say this has occurred with others as well.
