Listing standard gold piece values in the first edition Dungeon Masters Guide was one of Gygax's worst decisions, in my opinion. From there it's only a short leap to shops that stock magic items nas standard wares, which quickly pushes any pseudo-medieval gameworld towards an implicit technomagical industrial revolution. And finding a magic item yourself (especially if you were nearly killed by gods know what getting it) will always beat the pants off of picking up your new vorpal blade at Sears Roebuck.
There's a third option besides buying and looting that I wish there was a little mechanical support for: the magical gift. Think about the Lady in the Lake and Excalibur or the Phial of Galadriel. Wouldn't it be cool if there was some sort of good, solid criterion for "You are worthy of your quest. Take this, as it may be useful on your journey."? 4e's Treasure Parcel rules (which I otherwise loath) might actually be on to something in this regard.