Legally it's an allocation, not ownership. The registry could simply revoke your allocations if it caught you selling them (though whether it would is another question).
Exactly, its more like a lease. You can't actually sell an IP because you don't own them, but you are given a lease/license to use them.
Frankly, the only way to truly transfer an IP block between companies is to have one company buy the other, then the IP addresses can be reallocated to the new owner. You can't just transfer an IP block to another entity according to ARIN's registration agreement.
When I owned an ISP in 1996 I paid a larger ISP money to suballocate a range of a couple /24s to me. This happens all the time - look up any random netblock on arin.net.
It depends on the policies of the RIR (Regional Internet Registry) in the region where the IPs are from.
That said, if you're willing to ignore the RIR's policies (which a lot of people do) you can usually "black market" trade IP addresses, so long as you can convince your upstreams to accept them (which they usually will).
