If it hasn't been shipped, request that the seller cancel.
If it's been shipped, verify authenticity with crisp clear pictures. (It could be that this seller mixes authentic and fake and this could possibly be a genuine item.)
If it's deemed as fake, you can open a SNAD dispute for not authentic. Seller will be required to supply return shipping label at her cost and you can return. (It MUST include signature confirmation.) If seller denies the return or doesn't respond to dispute, escalate and let ebay close the case.
Don't lose sleep stressing over this. As unfair as it sometimes seems to sellers, ebay is considerably pro-buyer and you won't lose money.
Not always, I had an item that was marked as shipped but never showed up, I asked the seller a few times if she could check, she told me to check with the post office, without any tracking, not possible, she claimed tracking wasn't available, which is rubbish, anything you post in the UK, you get it tracked for free (too many things disappeared into nowhere), after explaining that I called the post office and they went "Without a tracking number, we can't help you" she got angry and told me I have it, I told her if I had it, I wouldn't have asked her to check where it is. When she told me to eff off, I opened a case, I don't know how she managed to escalate it, but it was closed as apparently I had confirmed receipt, which I hadn't, took me a good while to get through to eBay chat, requested that they look at the messages, that I had not confirmed receipt, that I have not received the item but would really like it, that every item has a bar code that is scanned when purchasing postage and along the way, and again when delivered, after 3 additional days I got refunded - the dog bed never showed up. With an item that size, there is simply no chance that somebody slaps a stamp on it and puts it into a post box (apart from stamps hardly being used anymore), but sometimes somebody with eBay chat seems to not grasp the language, so I would recommend doing the return, but apparently there are now scams where people do not take returned items (not in, or a premise that doesn't exist) so you can't show you have returned and it gets back to you, eBay chat help doesn't allow you to upload pictures so you can't upload the "Address not known" return. I am not overly paranoid, but everything over 200, I only use the PayPal credit card option.