It won't go well unless you are this type of LL and accustomed to dealing with druggies and criminals.

I'm wondering what services you all use to screen tenants in Seattle, in order to conform with the "second chance" law preventing criminal background checks (or even asking questions about criminal history). 

Some are drawn to the criminal lifestyle, some got on the wrong track for a only a short while, and some made one mistake that cost them dearly.

Criminal element handling depends on the situation.

Regarding any potential legal action, I come from a law enforcement background, so I am trying to look at it from a criminal and civil law issue.  

Including a special discount on Tenant Screening service. http://www.biggerpockets.com/perks This is a REALLY good service and includes the criminal background check.

The service I use does credit, past residences, evictions, and criminal background all at once.

Stick to the information on this site and save your money from criminals.

He was concerned that merely having a signed purchase contract without offering them the right to purchase would put him in violation of the law and open him to civil/criminal penalties.

It is always my preferred course and usually what I advise but in this case he probably has enough on his plate without also learning the process of bringing what I label as criminals to justice.