Hey there, welcome to the forums! There's a place to ask for price checks in the trading section, but as you still don't have access to it, creating a thread should be fine.
Anyway, I think this difference exists because this is a HTS item. It may be worth 15m, but if nobody wants to buy it for this price, then its value is decreased. Law of supply and demand applies here.
You will probably take a while to sell this for 10m+, that's why some people prefer to sell it at a lower price (in your case, 4.5m) to someone with more patience to sell it. I personally would take the 4.5m offer because I'm impatient selling HTS items, but if you think it's worth the wait, then wait, but keep in mind there's a chance that it will never sell (especially considering how dead the game is).
Edited by Hikimaru, 29 April 2017 - 07:31 PM.