Like the title says - when buying a site, do you guys have any good sources/tools for verifying the site to see if it has been blacklisted/banned by the search engines for whatever reason?
It'll show lots of info to give you a good idea whether the domain had fallen foul of the SEs.
For example, if a domain is over 6 months old, doesn't have any indexed pages (even thought there are some on the site), no PR, a few incoming links - it *may* been banned.
Use a little common sense and some guesstimation and you should be OK!