All items you find that have a base value of 1-5 bottle caps are worth three (five in J.E. Sawyer's game) times as much when you trade them.
Example: Frank searches every container and corpse he comes across. A lot of them have tidbits of old world nostalgia or technology that's effectively worthless and practically only worth a few caps. However, most bits weigh almost nothing, so he collects it all, knowing that he can sell even a broken little doll to some poor sap for a decent amount of money.
The perk description was posted by J.E. Sawyer on the Interplay forum in 2003.