I am of the opinion that artifacts should never be purchasable, sellable, or included as part of a character's starting equipment, regardless of level. They are story devices. If your player wants a hammer of thunderbolts, perhaps make the quest of a hammer of thunderbolts part of the campaign. Have the player pay 10,000 gp (or whatever you feel is appropriate) for a map to a dragon's hoard which allegedly contains a hammer of thunderbolts. Little does the player know that a group of storm giants are also in search of the hammer and the PCs will have to contend with both the giants and the dragon to acquire it.