Barbs are nippers, especially when they're setting the pecking order.
How big is your tank? They need a fair bit of room to swim. If it's not there they will get aggressive to each other.
You said your tank is new, is it cycled? That will add to the stress as well as their mortality.
As far as the LFS
taking the fish back, you'll just have to call. If the fish is in good shape they might.