a) "black markets" in (human) eggs might explit the poor or vulnerable.
b) it is an invasive procedure to remove the eggs from the ovary. Damage may be done to the ovary, thus restricting the female from conceiving at a later date.
c) women may be commissioned (paid) to provide a supply of eggs.
d) there is only a limited supply of eggs provided by IVF (in vitro fertilization) clinics. Some of these eggs may have genetic diseases.