I have a condo in South Beach. If I list flat fee in the MLS and I find my own buyer, will I pay any commission?

No! You will only pay a commission if a licensed real estate agent finds you a buyer and the buyer actually closes on your property. You can market your home as a FSBO at the same time through any other marketing media. Note: MLS rules do, however, prohibit you from advertising a different price from what's listed in the MLS.