Questions about the IPv4 block market and IPv4 block leasing.

I been trying to research the IPv4 block leasing market, I couldn't find much info about it. I been told it's a bad idea to lease out IPv4 blocks. I also been told the major problem with leasing out IP blocks is spammers. Hopefully someone here knows something about it. I'm not talking about ISPs leasing IP blocks to their customers. I'm talking about 3rd parties leasing to people for use on their ISP or their server host.

1) What is the market value of IP blocks that have been used for spam?

2) What percent of the time are leased blocks used for spam?

2) What's a normal rate of return for leasing out IP blocks?

4) Generally, how long are the leases?

5) How big is the leasing market?

6) Is there anything else I should know?