Bulk Trader (Fallout Supplement)
From D&D Wiki
Quantity is the game, and you're the master. When you buy and sell goods, you get a better deal for larger quantities of a single item.
Prerequisite: 4 ranks in Diplomacy
Benefit: +25% more TUs/Caps on selling five or more goods at once, or -25% less TUs/Caps on buying five or more goods at once.