I can't say whether you've been scammed or not because I haven't seen the packaging and
decals in person, but you have to be aware that many things on eBay are just counterfeit items.
Some you'll never know unless you've got the real deal next to the dodgy item and compare the two under magnifying glass, because they're getting really good at copying stuff, but there's also the really dodgy stuff that's way under par with regards to quality...
Have you ordered
decals from the same manufacturer before and kept the original packaging to compare with, and to compare the actual
decals and the look & feel with?
Check the packaging for spelling errors or other signs of it not being original, like low res print or "generic" packaging.
You can buy blank decal sheets and print your own at home, which I'm sure are also used to make counterfeit
decals of varied quality for a quick scam.
You can always do a complaint/refund in eBay and see what the seller does?
What's his/her score on eBay? Checking what people have written about a seller is important before making a purchase because you can easily spot the fake reviews - and that's a sure sign to stay away!
There's also the route of just bad luck, with a bad batch, but if there's a coin to be made you can bet there's pirated versions available to buy on places like eBay and alibaba!
I've stopped buying things like this on eBay, unless they're a real and serious business that wouldn't be so stupid as to sell counterfeit items. Sadly sometimes even they are tricked, but this is less common - especially if you only buy from someone reputable.
So check the goods if they could be counterfeit and then contact the seller for a refund, or a replacement - for free obviously.
If counterfeit there's a process on eBay for reporting this.
I've been had on more than one occasion but I always make sure they get reported, otherwise they're free to continue to scam.
I hope you've just been unlucky and that you have a seller that will set things right for you!
Edit: the above rant about counterfeit eBay items are more or less only applicable if the seller is from mainland China or Hong Kong. Not common in Japan, US or Europe. Eastern Europe can be tricky because there are counterfeits there - but perhaps not in scale modelling? We have very strict laws regarding counterfeiting whereas it's more or less legal in China if it's a "limited run".