Although I don't particularly like the Oath of Greed (it just isn't "heroic", although since Cavaliers can be any alignment, I guess thats ok), I do have a problem with the mechanics.
It requires the character to come into possession of a single item worth 1000gp per cavalier level, and provides a +1 bonus to three skills (+1 ever 5 levels). So to get a +1 bonus at level 3, you have to find a 3000gp item? Maybe I'm just stingy as a DM, but I don't think most low level characters are going to stumble across expensive items that they can "hold hostage" for a +1 skill bonus.
And the cavalier keeps the bonus if he sells the item for "proper compensation"; for how long? I'm assuming til the end of the day, but technically hes still keeping his oath (unless he just ran out and spent all that cash at the tavern).
Definitely not an oath I would allow a player to use, although maybe an NPC....