I have been obtaining open code enforcement records from cities in my area and sending out yellow letters for marketing and I was wondering if there were any other good and accurate methods that I could use. 

If talking to them or their landlord does not solve the problem, consider a call to code enforcement.

Although permits may be revenue generating for the city, they are there to actually protect the property owner/investor, homeowners/tenants and contractors from hazards by enforcing the adherence to the conformity which the city has made the standard.

Find the confidence to lead the process and the moxie to enforce the terms of the rental agreement ongoing.

BEWARE: Please be advised that in Maryland an email can be an enforceable contract in some cases if the buyer was feeling especially froggy.

There will always be losers in any legislation change.

It's either that or they sign a lease (which you've also had reviewed by an RE lawyer) so that there is a legal basis for enforcing regular rent payments, and then those recently-created tenants do, in fact, start paying rent.

When you say 'request', it sound like you mean 'enforce'.

Then they whisper to the buyers what a terrible house it was, the heating system doesn't work, and they plan to call it into code enforcement about other violations.

But, you don't want to go through the headache of trying to enforce your contract, if you can help it.