Purchasing IPv4 space - due diligence homework

["Torres, Matt via NANOG" <nanog () nanog org>]

All,
Side stepping a migration to IPv6 debate.... I'd like to hear advise from the group about performing due diligence 
research on an IPv4 block before purchasing it on the secondary market (on behalf of an end-user company). My research 
has branched into two questions: a) What 'checks' should I perform?, and b) what results from those checks should cause 
us to walk away?

My current list is:

  1.  Check BGP looking glass for route. It should not show up in the Internet routing table. If it does, walk away.
  2.  Check the ARIN registry. The longer history without recent transfers or changes is better. I don't know what 
explicit results should cause me to walk away here.
  3.  Check SORBS blacklisting. It should not show up except maybe the DUHL list(?). If it does, walk away.

Anything else? Advise?
Thanks,
Matt