Is there a sin or expiation for us for accidental killing, given that we transported weapons and ammunition that were used in fighting between Muslims, knowing that we did not directly participate in the battles?

There is no objection to selling or transporting weapons if the fighting is legitimate, for the sake of establishing justice, as opposed to fighting during times of discord (fitna) or aggression. One must consider the party to whom the weapon is transported; if they are just, then the transport is permissible, but if they are oppressive, then the transport is forbidden. Repentance is obligatory for whoever assists an oppressor. The consequences of killing fall upon the one who directly commits the act, because direct action takes precedence over the cause.