Unsolicited emails must comply with the
CAN-SPAM Act.
Your brokerage should have a do-not-email list (since the opt-out will apply to your entire firm, not you) and you can't email anyone whose address is already on that list.
There is no law that I'm aware of (but I'm of course not a lawyer, and local laws may vary) that would outright prohibit you from using the FSBO site as a source of leads. However, that site's terms of use may prohibit you from doing so.