Yes, you can open a real estate agency in Thailand, if you have a standard Thai business that is owned 49% by you.
However to trade in land is prohibited under Thai law by foreigners.
If you are just going to be a service that rents/sells condos as an agent, then no problem. You should partner up with someone here to has a real estate license to make things easier for you.
Starting a real estate agency is a GREAT idea and we wish you the best of luck!