You have mastered one aspect of bartering - the ability to buy goods far cheaper than a normal person. With this Perk, you get a 25% discount when purchasing items from a store or another trader.
In Fallout, this perk adds the equivalent of 25 invisible skill points to the player's Barter skill when determining prices. 
In Fallout 2, due to a change in the Barter formula, this perk subtracts 25 points from a barter modifier, which is then multiplied by the original price and the Barter skill modifier.
By default, the modifier starts at 100%, but Reaction modifiers can add 25 points, 0 points, or subtract 15 points at the same time, making the discount appear to vary from 20% (when there is a +25 point "negative reaction" modifier), to 25% (with "neutral reaction" modifier (0 points), up to 40% (with a -15 point "positive reaction" modifier).
Allows for expanded dialogue options when discussing business arrangements outside of the barter system. Especially applicable when speaking with Uncle Roe of Canterbury Commons, see Merchant Empire for details.
This perk only affects purchases, one's own sold items' price are not affected by this perk.
If the 25% discounted price is below base price, the list price will be the base price.
Once this perk and the 60 Barter necessary for it are obtained, most vendors will already be selling their items at the lowest cost (Base Value).