The egg and sperm donors. 
In the late 80's I met one of these incubator for hire, my sons best friends girlfriend.
I didn't know what to make of her a lovely 25 year old woman, college graduate, a potter by trade very talented.
This was her 3 baby and most likely the last for her to sell to desperate family's. She told me she received all medical bills paid for including naturally the procedure. All living expenses, food, clothing and shelter and when the baby was releases she would get $10,000 for her time and trouble. This was at a time when the minimun wage was $5.11.
Why did she do this, she made a good living teaching pottery classes ??? Some times the great mysteries in life boggle my mind.
Nothing immoral about rent-a-womb, Today with science some mothers in their 60's are carrying their grand children or one sister for another. Once the sperm and the egg meet up in the bottom of a test tube there is life, this must be protected some how and if the egg mother cannot carry a baby due to medical problems, that's when they call in Wombs Are Us.
As she told me she was just baby sitting for the biological parents.
The contracts she signed for this procedure were long and filled back then with 100 clauses of any reason to break the contract. If caught smoking, drinking or doing drugs, the contract was off. If she had a miscarriage, or the baby was found to have sever health problems the parents could call off the contract.
The owner of the womb signed a contract to deliver to the parents a healthy baby of either sex period. If for any reason her fault or the fault of the genetics of the child from the parents, to carry it or abort it was now up to the owner of the womb.
Now to moral and leagel issues, this womb refused to abort the baby, the genetic parents turned their back on their own child. The womb delivered a child, in no way related to her and child was placed for adoption.
The parents have some where out there a child that may some day with science get all medical problems fixed, were I one of the parents I would worry about that knock on the door 20 years from now and with DNA records demand a big cut in the family's inheritances. [ Has to be money in the family to rent a womb ]