Minecraft account no longer exists?

When signing into my minecraft account it logged me in on a demo version. I tried logging into minecraft on their website, comes up that my account and username doesn't even exist. Signed in on my mojang account, doesn't come up that i have minecraft as a purchased game. I paid using a gift code that i bought off eBay; it worked fine but obviously my minecraft account suddenly no longer exists. When checking to see if my minecraft username had 'paid' on the database, it came up false.
What i'm guessing is that the eBay seller has refunded the gift code so they can go on and sell it to someone else; is this a possibility? And as it's after the sale and it did work, i pretty much can't do anything to the seller.
Can you refund or reset minecraft gift codes in order to use them elsewhere? And what other reasons could there be for my account no longer having a minecraft account…