GHD products are being sold exclusively on their official website, Link hidden: Login to view
and through approved online retailers. If you go to this page, Link hidden: Login to view
and enter the two retailer websites you have mentioned above, you will find that they are not approved by GHD which means that you might encounter issues with authenticity if you deal with them. I suggest that you check out the link I have provided above. That page does not only allow you to verify your supplier but it also lists approved GHD online retailers that you might be interested in.
Be careful with fakes. If the price is too good to be true, then, it might be. There are a lot of fake GHDs online and even on eBay. Though eBay does not tolerate listings of fakes, they don't really have 100% control over this and every now and then, you will find some fakes in the site. This will continue if not reported to eBay.
Did a quick check on eBay and some market research tools and found that it's quite a big niche market. I checked GHD Hair Straighteners specifically and found that the monthly searches for this product is quite high. The sell-through rate on eBay is also high, at about 79% .
Hope that helps!
All the best,