Prior to closing on the home owner had to legalize everything with City and include revised Certificate of Occupancy (CO). 

People who understand science and tech will be in high demand for the foreseeable future.On the flip side, while you are doing your flipping gig, pick up certifications like PMP, which are in high demand and will keep food on your table if the flipping thing doesn't do well for a few years. 

My contractor told me that we needed a mold certificate before he could close up the walls and that it would protect us from any liability when we sell the home

As a Chemical Engineer, I researched my first mold property in 2003 adn realized that there is no official certification for mold remediation - only training classes that allow you to say you are certified and hang your shingle out the next day.

David Krulac - Yes, you can't get a "Certificate of Occupancy" with those locks, but while vacant it's unoccupied, so IMO the certificate can wait until I've got an occupant (tenant or buyer).

Supplemantal Directive 09-09Notice that the sale must represent an arms length transaction and that the purchaser must not sell the property within 90 calender days of closing, including certification language regarding the arm's length trancaction that must be included in the sales contract.Has anyone run into any issues regarding this in an A-B-C transaction?

I was also in the process of learning about tax lien certificates and was going to expand into that area of investing as well.

Why are they not required to learn about real estate investing as part of their certification.

They are as official as they are the paper you issue the shareholder certificate on.

Is there special certifications you will need  to be a land lord?