Haha, right. Are you even thinking of the communication side of this?
You know that's what it means. I know that's what it means. Probably every man jack on this board know that's what it means.
On the other hand, 99.9% the millions of TV viewers watching a game who get a five-second shot of a banner saying "Full Sale Only!" probably does not know that's what it means. And that is very much the supporter's trust problem, because what they'll think (reasonably, lacking the full context) that means is "100% of the shares", since this is normally the meaning of "Full Sale". And since probably quite a few of them also knows that the Qatari bid is the only one to have that element (which requires no more than generally following news bulletins about United), they'll also (and again reasonably) conclude that's a statement of support for the Qatari bid.
Point: If you don't want people to think that, don't put "Full Sale Only", but find another way to express it. That's all.