Repairing is not free. The more durability points that must be restored, the more expensive the repair is. Ethereal items cannot be repaired. Some items repair themselves over time.
NPC that can repair itemsEdit
Diablo I and Hellfire Edit
Diablo II and Diablo II: Lord of Destruction Edit
Repair in Diablo 2 Edit
In Diablo 2, repairing is the means by which typically costly spell charges are restored to equipment.
A Ral and an armor piece combined in the Horadric Cube will result in the armor being repaired. Orts can be similarly used to repair weapons. As the repair cost of very high level armor can be many thousands of gold, it can actually be easier to trade for a Ral than collect that much gold.