Sure, adoption markets basically already exist, why not make them legal?

Not only are wealthier people better candidates on average because they can provide for the material needs much better and will on average have a more suitable psychological profile (we can impose legal screening of adopters too, so they need to match other current criteria before they can legally buy on the adoption market if you feel uncomfortable with "anyone can buy"). It also provides incentives for people with desirable traits to breed, far more than just subsidising them having kids of their own.