I had this problem earlier, but can't now replicate it and am able to choose how many items to buy in both the Trade and Barter views.

If you have a save from which you can reliably repeat the problem, this seems definitely worth reporting to Larian. You can use the "report a bug" function on the Larian launcher as a shortcut to the relevant part of their website.

"You may call it 'nonsense' if you like, but I've heard nonsense, compared with which that would be as sensible as a dictionary!"