Very simple to check
For Amazon: Follow the pictures in this post, if the status is "just launched" that means this seller is not reputable and it's a waste of time and possibly money to buy from him especially if the price doesn't make sense.
Same thing with eBay, if the seller doesn't have at least 100 Seller feedback (make sure it's recent feedback, will explain in picture) please don't post here as it's most likely a hacked account and you will not get what you ordered and possibly struggle with eBay/PayPal to get your money back.
Look at the first comment for eBay pictures