I did some analysis once about a year ago, and found that a large number of domains were inactive.

http://garysieling.com/blog/one-third-site-auctions-abandone...

It's possible this analysis is over-estimated (e.g. could be defects in the scraping process) but I was surprised even just checking domains randomly how many were later abandoned.

I suspect that there are a lot of sites that use various techniques to fake their numbers; if this gets the purchaser banned from adsense it makes a site in the lower price ranges basically worthless.

If you just look at the auctions from a year and go and click on a few of them.... almost all of them are dead.

I kind of think this indicates this isn't an efficient market

Are adsense bans forever? Are they tied to an identity or a credit card?

As far as I know they are pretty much permanent. I was banned about 2-3 years ago and I have sent an appeal every year. My appeals are always rejected.

Accounts are probably attached to your identity (SSN, domain, etc.)

The only way to get another adsense account is if you form a new legal entity.

I found that the only way out of the Google perma-ban was to grow large enough to have them call you instead. Their representative - apparently surprised by the fact that my site was banned - clicked a couple of buttons and the ban was removed. The account even had the hundred dollars that was there when I was banned years earlier.

How big do you have to be?

I don't want to be too specific, but the the site yields Google more than 10K USD a year -- which I guess is incentive enough. They still keep pestering me about putting more ads up, tho.

Seems to be. In my experience it flags the domains, not just you.