Well, few problems here.
First Sythe isn't the Runescape forums, so you cannot trade an item for purely RSGP/RS items here. You do that in the RS forums, we cannot help you if you aren't a member.
2nd Purple won't sell for mid, but you already know that after trying. Purple is around 790M street, so it's slightly below mid.
3rd A red is around 1.09B, it's 300M more than a purple. Not only are you trying to make Sythe your Runescape Forum, but you're basically trying to merch.
My advice is to determine what the lowest selling Purple price is and undercut it. Or wait till Purple goes up till you're satisfied, but don't expect it to. I suggest you take a look at "R Quark" FC for frequent price updates on rares which includes the phat you're trying to sell and looking for.
