I got a spam email to one of the addresses I don't use; all that address ever gets is spam. A couple of decades ago, it got onto a list of email addresses, and it's been sold and resold endlessly ever since.
This time, the spam was from a vehicle leasing company. They thought it was a business address (it isn't) and that it had opted in (it hadn't). I told them this, and the spammer told me his tale of woe.
He bought a list from a Nigerian (not all Nigerians are crooks), 600,000 addresses, and he spammed to it. He got 10% failures to arrive, and 10% unsubscribes. I didn't point out to him that most people being spammed don't bother to unsubscribe, so 10% is a big number.
So, he said, that went well, he even got some sales leads. So then he bought 1,200,000 addresses from the same person. What he got, was the same 600,000 he already bought, and an additional 600,000. So he pointed this out, and was told "I never said they'd be different", which is maybe true, but it's kind of implied in the transaction. So he refused to pay the invoice, and only paid half, which sounds reasonable to me, but the vendor was very angry, and called him names and made threats.
Then he told me the name of the list vendor, and it's a company I've met before. And the name of the guy running it, also rang a bell. I looked him up in my records, and sure enough, there he was, and it was also in connection with vehicle leasing.
So I phoned him, he was on the same number as before. As soon as I started to tell him the problem, he remembered me, He said he'll look into it, but I'm not terribly optimistic.
The business of selling email lists, is full of people with doubtful ethics. You could say that this servers the buyers right; it's the buyers of lists that are actually doing the spamming.
So if you're in the vehicle leasing business, and you're thinking of buying a mailing list, contact me first and run the name of the vendor past me, and I'll tell you if they're known to me.