The problem right now is players can not repair other players gear with dark matter. You can buy dark matter from the grand companies with GC marks instead of gil. In 1.0 they had a pretty nice repair system where you can put your gear up for repair and the crafter repairs it using your dark matter right out of your bag. You pay an automatic nominal fee per repair which would be 50 gil on grade 5 darkmatter repairs in current game currency to the crafter.