The seller must have a signed Listing Agreement with a Participant to place a listing in the MLS. If a Listing Agreement is in place, the house is not considered an FSBO.
If you are on the Buying side of an FSBO, you can enter it into the MLS with a signed Sales Fee Agreement if entered within five business days of the close date.
To enter a listing of this sort, you will act as the Listing Agent (even though you are on the buying side), select Sales Fee Agreement in the Listing Agreement Type, and have the public remarks read "Entered for Statistical Purposes Only." You will then pend and close the listing on the same day as entering the listing. You will use "nonmls" as the Selling Agent ID (this way, the listing is still one-sided).
You may not enter yourself on both the selling and buying sides. If you are on both sides, then the listing is not considered an FSBO and needs to be put in like a standard listing as an Exclusive Right to Sell promptly after signing the contract.
[see: 2.2(e) MLS Technology, Inc. Rules & Regulations]