On the one hand, a womb is an organ, just like the heart or lungs. If there was consent, then it's ethical.
On the other hand, given the abuse we see in the surrogacy world, I can see this being the start of something much worse. We already know about the imbalance of power, sexism, class and money and what it does to poor women. We've already seen doners die from carrying surrogates. What's to stop the Iranian regime from selling the wombs of executed political prisoners? Or girls in Afghanistan being handed over for donor surgery because their families are desperate and they have no other choice?