You're asking about whether it's "illegal," but I think the real
"underneath question" is actually "will we get sued?" Illegality relates to criminal law. But as you probably already know, just because something is legal
, that doesn't mean you won't get sued. People end up in civil court all the time over things that are not, technically, "illegal."
So what you really need to do is consult with a lawyer. They can tell you not only whether what you want to do is "legal" (I think, assuming you don't misrepresent your involvement in the sale process, it probably is... especially if in your area -- like where I live --the terms of sale of real property is a matter of public record), but they can also help you estimate the likelihood of facing a civil suit, what your chances would be of coming out on top, and what specific steps you can take to reduce the chances of being sued and increase the chances of winning in case you are sued.
If you're at all concerned about the legal risk of doing (or not doing) anything, it pays to consult a lawyer, rather than trying to figure it all out yourself. Assessing and mitigating those kinds of risks is a large part of what they do. It's kind of like going through a realtor versus trying to "sell by owner."