Well, at least (at some point, I'm not clear) they disclosed it was inauthentic. I'm not sure you'll be able to send it back UNLESS (authentic and/or branded) were in the listing. I also don't know if PP or whatever will support a chargeback to the vendor (they may or may not) given it's vape gear, which is (technically) not supposed to be sold on ebay, although it is, all the time and etc.
I bought one (low end, not used) mod on Ebay, never again. I kind of chalked it up to my own fault, and proceeded from there. I don't buy much at all vape related on ebay, maybe vape skins or something would be okay.
If you can get your money back, well I would try. I am doubtful that a smok knock off is going to perform well and may well be dangerous. Do check out legit vendors on the site for your next purchase. You may well not get the "incredible awesome" price you might on certain sites, but there are usually REASONS for that and etc.
I'd try for a refund, but personally I wouldn't vape it. There are some legitimate vape stores on ebay I think but you have to find them, identify that they are genuine manufacturer sites, and make sure they have great feedback and etc.
I sympathize .I made my unfortunate ebay purchase, (vape related) when I was relatively new. Don't give up, find a better vender and some of the Chinese vendors offer some great prices, at low rates, especially during sales or in clearance.
If you know what you want to buy next, you might post it and get suggestions of where to pick one up at a reasonable price. Best of luck,
Anna