I dont know of any used ones for sale, but have you seen the new Diamond bows. My girlfriend shot one the other day, first compound she has ever shot, she said it was way cool. They are made to be adjusted as the kid grows, and for $35 you can have the limbs upgraded when she needs more poundage. I dont know what kind of mone you want to spend but this particular one was $380 and it was ready to shoot, factory sights them in also.
Brandon