I thought this was quite an intriguing premise, birthright or birth theft.
First of all being able to buy a new life to extend the one you have and being able to pick whom you are born to. Of course it also means the baby you steal the life from has to go somewhere else.
Instead of their planned life they are subjected to a life of negativity, bad choices and in general an abysmal life. The thief gets a life of positivity, riches and pleasure.
The so-called Unborn are marked and identifiable, which makes them easier to hunt and destroy. Sometimes the ones who have taken their place need to ensure there is no threat to the life they have stolen.
Gage has created a fascinating concept with a lot of possibilities for further development. If that same level of creativity is applied to the character development and the rest of the story then this will be a cracking series with a lot of potential.