I'm trying to find out if it is OK for me to aggregate commercial real estate listings from individual brokers sites and put it on my site (with all of their information on their correctly, phone, email, website, etc). The objective being to create a source for niche commercial property buyers to use.
If you simply aggregating information than I don't see any issues with copyright. But, I would seek permission from the brokers/website owners to make sure business is protected. It doesn't hurt asking for permission, and if it directly benefits them to get leads/sales than none of them will say no.
I also cannot speak to the legal aspect, as I am not a lawyer. However, as a web consultant, I have many clients who use similar strategies from product resellers to realtors.
Real Estate has a system for this in place, the "multiple listing service" (MLS) - http://www.mls.com, which allows you to aggregate real estate data from other brokers.
However, one thing to consider in this area is the effectiveness of this strategy. If you are not producing any unique content, but instead only reposting content from other sources, then the search engines could see you as a "content farm" (http://mashable.com/2011/02/25/google-content-farms/).
If you want to show up in search results and bring in traffic to the site, then you will need to be producing some sort of additional unique content of value. Be that in a blog or other info added, you will need a content strategy that goes beyond reposting content from other sites, or the search engines are going to punish you accordingly.