This applies to all five haggling perks.
Quote "Buying and selling prices are x% better."
This applies only to the sell price vs the unmodified price. The buy price vs the unmodified price is always less than the x%. An example to illustrate.
Perk: Haggling00
Sell price modifier: 1.10
Buy price modifier: 0.91
With an unmodified item sell price at 100 septims, the result is 110 septims received. That's 10% better.
With an unmodified item buy price at 100 septims, the result is 91 septims spent. That's 9% better.
nWith Haggling80, the same application gives a 30% better selling price, and a 23% better buying price.
They screwed their math and made one the reverse of the other.
Ignoring for a moment that money in this game is far too easy to come across, the result does not match the specified intention.