Typically by-the-book mainly because there are often a lot of items and we dont want to take the time to haggle with each merchant. However, exceptions do occur. When the players have done an exceptional task for a merchant or highly placed official they might recieve a letter that lets them sell stuff for 75% with the local merchant's guild. Occasionally items have a value beyond their obvious worth as collectors items and the PCs can get list price or better for them. When buying PCs often get somekind of discount when working with a temple, guild, etc for mundance items and low level potions and scrolls. Recently a PC asked an artificer of a relgious order about making a +1 Fiend's Bane Long Sword and given the nature of the order, the PCs relation to them, and the circumstances he was able to commision it at only slightly more than production cost. These, however, are rare exceptions.