She has powerful lifeforce, that can heal wounds and replenish chakra, but she doesnt know any healing jutsu to perform, therefore people need to bite her. http://naruto.wikia.com/wiki/Heal_Bite
And even though wounds of other people, healed by healbite, get healed rapidly and leave no scars, she is left with all kinds of bitemark scars (on her neck, arms AND on her CHEST (I wonder who made those bitemarks :p).
Kabuto, someone who took her ability (by injecting her blood), could use it to heal his wounds (no scars either). When Naruto went into 4tails mode that destroyed his skin, he wasnt left with any scars and the kunai he plunged in his arm in part one, didnt leave a scar either. For some reason, Karin is left with bite mark scars, while she is suppose to be one of the best in terms of regeneration.
I believe its because her chakra depletes after someone bites her, not allowing her own wounds, the bitemark wounds to heal in time, leaving scars? What do you think?