You can't buy/sell/trade "ASN blocks". The only people handling "ASN blocks" are the 5 RIRs (APNIC, RIPE NCC, ARIN, AfriNIC and LACNIC) and IANA.
> owning IPs is the ultimate rent seeking cloud business
It also seems that your use of "rent seeking" doesn't match established use. It normally refers to people extracting money for things far beyond their actual value. The IPv4 market is working pretty well on a supply vs. demand price feedback loop, i.e. the prices are in fact just reflecting the scarcity of IPv4 addresses. The term "rent seeking" does not fit that situation.