Unfortunately I am 100% sure, having in fact tried it out myself (singleplayer + fly is a good way to test collision) and having a rudimentary understanding of how UE2 works in this regard.
From the docs:
bool bBlockActors
Blocks other actors that also have bBlockActors set. The two actors will not be permitted to occupy the same space, and if the location of one is changed such that it would overlap with another blocking actor, the first (the one that just moved) will be moved to the nearest valid location by the engine. This defaults to false.
The key word in there is ACTOR. Nowhere is pawn/player mentioned, and in fact it could not be, given this is a property of Actor not of Pawn.
In case you weren't aware this is the base class of ALL objects in the game world and therefore include ladders, blockingvolumes and everything else
by definition.
Essentially the game engine does not support specifically setting player vs player collision at this stage, hence my previous post calling for TWI to add it.
EDIT:
Don't get me wrong it's a real shame your solution doesn't work perfectly, if anything I'd want it to work more than you! (imagine 32 players in an undersized trader) But a solution to a problem can't go creating more...
On consideration a more refined reproach would be to only disable collision if a player is penetrating/touching the Trader volume, which would mitigate this issue outside the trader, but then there is still the issue of the blocking volumes used to stop players climbing behind the counter and getting stuck.
Best solution is if TWI added a flag to KFHumanPawn setting whether it collides with other ones or not.