(several days ago they were high alching for 9k but now they only alch for 8179 so its no longer a viable way to make money and if someone wasnt paying attention they could lose a lot because it would end up losing like 500-600gp each ish.)
Enchanted onyx bolts have a Life leech special. This gives the bolts a chance of doing extra damage (20-25% chance from testing) and healing the attacker's Life Points by 25% of the damage dealt. This effect does work with abilities. This does not work on the undead, as they have no "life" to leech.