I guess better barter breaks the game in the sense that it disrupts the general flow of progression by allowing people to get end-game gear without doing end-game quests. But then you can get end-tier loot 60 levels early just by going to the wasteland, and you don't even need to do quests--just nerd-pole to the POI's last room, loot the stuff and come back when loot resets.
The key thing is that 7DTD is just as much a sandbox game as it is a survival game--you have freedom to play how you wish, and set your own limits/restrictions as you see fit. You have multiple options for obtaining loot, and using better barter is just one of them. Better barter is pretty much always the best one as I have discovered through numerous playthroughs, but then how do you balance it? Make items more expensive? They already did that, but then that means that if your playstyle involves NOT teching into better barter, you suffer from expensive prices, just as much as the better barter players do.
A possible solution would be to reduce the chances of the good items appearing in the secret stash, even with better barter.