Why is it illegal to sell your children?

I cannot put an ad in the paper offering my 2 y/o for X amount of dollars, BUT I can offer to be impregnated by someones husband and carry a baby for them to term in exchange for living expenses and monetary reimbursements. OR I can find myself pregnant and decide to search for adoptive parents who will take care of me until the child's birth (and you just know that some of these people are giving extra money out). So my question is: What's the difference?