[mfincham]

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).

[duskwuff]

Many of the legacy allocations were made before RIRs existed, so no policies really apply.

[mfincham]

Sure, but when you want to update the WHOIS / reverse DNS or whatever you have to convince an RIR. If you don't care then I guess that's fine.