It seems to me that many up-scale merchants have the the immaculately attired manager who deals with the customers, sells them on the highest profit merchandise they can get away with and then directs the flunky to handle the filthy lucre, wrap the parcel and take it out to the customers conveyance or delivers it later. How would that be different than letting the alt do the actual transaction with a "very good sir, Henry here will complete the transaction for you. I must see to Lady Snootberger." Of course the customer can then have his man/alt deal with Henry too if necessary.
Of course you were talking about real minutes, your whole idea is to make thieving practically impossible. It would be better not to implement it at all than to implement it in a completely useless manner.
The server has nothing to do with role play. A chest knows nothing. At the very most someone might get a message telling them they noticed something but then it would have to be based on their relative perception and if they are facing the wrong way they should have no chance. Then if they do notice something exact enough to react to their reaction should be completely at their discretion -- no unsubtle hints from the server.
Of course realistically I could get three of my buddies to separate you from your chest by force if necessary but, oh! hey! that isn't possible is it? Just like with the way merchants put all their stuff out -- a dozen or two "Olivers" could clean you out and you might catch one or two of them but that is prevented by the server too, isn't it?
I got no more use for exploiters than you claim to have but I think you want to exploit your customers with no chance of retribution. Maybe you are all on the up and up and your prices are reasonable, as for me I do not sell much to other players, I give stuff away occasionally and sell ore very rarely. I am just as likely to sell it to an npc as another player.
If you are running a large scale business how much tax do you pay? I do not think 50% is too much for the octarch to take from the people who might try to replace him. All depends on how strong his guard is, I guess.