Thanks NANOG. For those interested, here is a summary list of due diligence tasks for purchasing IPv4 on the secondary market:

1. Check out the seller (Google search). What is their story? Generally avoid hosting companies because they may have more block/black list cleanup to do.
2. Check the BGP looking glass Internet routing table. Make sure the IPv4 address is not routed right now.
3. Check the historical Internet routing table. It is a good sign if the IPv4 network was advertised from the right company/ASN without recent changes https://stat.ripe.net/widget/routing-history#
4. Check ARIN registration. Singular, long standing registration for the IPv4 space is a good sign.
5. Check block list / black list. SORBS and Spamhaus are good resources. It is a good sign if the IPv4 network does not show up at all. http://www.anti-abuse.org/multi-rbl-check/
6. Check geo-location. It is a good sign if the IPv4 space shows up in the right global region. Try MaxMind.
7. There is a PowerShell script (that can be expanded on) to help expedite the checking process. https://www.saotn.org/powershell-blacklist-check-script/#more-2459

Thanks,

Matt