Humans are not pieces of property which can be bought and sold to the highest bidder. The main criterion for who gets an adopted baby shouldn't be who has the most cash to give the parents, but rather who will provide the best home for the child.
Well, adoption is basically the equivalent of like, the salvation army. You get rid of the kid ASAP, but you don't really get a return on it. Selling it would take more effort, but you'd actually get a return on your investment of time. I really do not see why you can't sell them. If the thing is the parent's responsibility, why can't they get rid of it in any humane manner? Why is it just adoption? If I got knocked up with a kid I didn't want, I'd want some financial encouragement to not have it aborted.