Well, what is triggering unit in this case? I think your trigger event is "A unit sells a unit." In this case the triggering unit would be the selling unit and the triggering player the owner of this unit.
Changing "triggering player" to "owner of sold unit" might do the trick.
Ghan has said he has fixed this. Monovertex please confirm this fix. This was only a problem with people that had signatures in the upper levels like not the special members but the respected members.