If a HUD-1 exists and has a person identified as the agent, or there is a settlement agent otherwise identified, then they are listed as the "Filer" in the first box of the 1099-S. If a HUD-1 wasn't used, or no settlement agent is designated, then whoever prepared the 1099-S is the "Filer" in the first box. Presumably you are the "Transferor" (seller), so your SSN is listed separately from the Filer's SSN.
See the section titled "Who Must File" on page 2 of the instructions for Form 1099-S at:
https://www.irs.gov/pub/irs-pdf/i1099s.pdf