It all comes back to supply and demand--and the inelasticity of each. If the demand is inelastic, then there is little incentive to lower prices and a huge incentive to raise them (Think adding another $2/pack for cigarettes--and you KNOW smokers will continue to buy nearly as many--inelastic demand).

So, if by lowering the price of a carboy $1, the dealer doesn't sell many more, and by raising it $1, he doesn't sell many less. you won't see the price change much.