So I got outvoted on this tonight, due to FFG-ness.
If a hunter is trading with another hunter, only those 2 hunters can talk in private, correct? They said that they can all freely talk, since it particularly says dracula can't listen.
Only the Hunters involved in the Trade Action can do this.
Rules say (page 13): The trade action allows two hunters to trade item cards and ticket tokens and privately share information with each other.
Regarding hidden information the rules say (page 9): Players participating in a trade action, or using Van Helsing’s “Leader” ability, can discuss strategies, share information, and view each other’s cards in private without Dracula hearing what is discussed or seeing what is shown.
It's just the two hunters involved in the trade action, not all of them, so your group was incorrect. Read the whole section on the trade action on page 13, and the hidden information on page 9. It's pretty clear it's just the two hunters involved.