Welcome aboard, ArChron
It is my best guess that the Live servers do more of a polling cycle that updates the clients as things are added or subtracted. I am confident the Emulator does not do this for various reasons; one being bandwidth. Not sure the original designers wanted their servers spewing tons of updates per second for merchants. I also haven't looked directly at merchant code, but I am sure it could be done. There are plenty of other things in the Emu that send updates to the client when things happen (like your buddy dying).
Maybe it's something we just haven't gotten to yet.
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