Van Helsing's "Leader" ability only says "when you trade", and not "when you do a Trade Action" or something similar.
So can, for instance, Lord Godalming declare a trade action while not in the same city as anyone, and then exchange event cards with Van Helsing? Or must Van Helsing be the one to initiate the trade?
I'm fairly sure the answer is that Van Helsing must initiate the trade, although the wording (as so often) could be clearer.
According to the reference guide for a trade action
When a hunter performs a trade action, he chooses another hunter in the same city as him. The two hunters can show each other their item cards and ticket tokens and give any of those cards and tokens to each other.
That seems clear that the person initiating the trade chooses and the target must be in the same city.
The reference guide also states
When using his “Leader” ability, Van Helsing can choose to trade with any hunter in any city on the board.
So in this case the choice is Van Helsing's which to me indicates he is the one to initiate the trade and so must be the one taking the action.
As I said the wording could be clearer but I am sure the intent is that Van Helsing must take the action.