Most of the nogotiations are, or should not be, a game, where you try to achieve advantage over the other "partners", but try an agreement that benefits boths sides, or all, sides of the agreement.
Beides, while at least telling your subjects what you are negotiating about, would not necessarily require revealinh all your cards, au contraire, public discourse may give you other leverage, or even more opportunities for bargaining.
I don't think there's downside in open trade negotaitions. Not open trade. Or, you know, open, free markets. There are many downsides to secretive, backhanding, misinforming, lying, deceiving martkets, though.