> What’s stopping a hedge fund from buying up a /12 block and renting it out?
These marketplaces are open to all and operate with the knowledge and blessing of the RIR's. However, the recipient does still need to justify the allocation they are buying just like when RIR's were handing out addresses directly.
While anyone with existing IP allocations are allowed to lease them out to anyone, even customers you are not directly providing connectivity to, IP's leased out to non-connected customers do not count towards justification for new allocations/transfers. This is different from Amazon or an ISP leasing IP's to their directly connected customers, which do qualify as justification towards further allocation/transfer justifications.
So a transfer would be denied by the RIR unless the recipient claimed they were going to directly use the /12 to provide internet service to themselves or customers. If they lied on the RIR application that would constitute fraud, which is one of the very few reasons RIR's will actively go and revoke a registration for. RIR's are aware that businesses and use cases can change over time and they explicitly do not revoke for simply not needing addresses anymore, to encourage a liquid and efficient resale market. So the question comes down to whether the application was in good faith at the time it was made. RIR's know what's up and it would be very unlikely for a hedge fund to pull this off without actually investing significant capital and effort to deploy an operational network that would justify a /12.
There have been proposals in ARIN to explicitly allow the leasing use case to count towards future justifications, or even for original allocations/transfers but these proposals have not gone anywhere.