I don't have the legal background that some of the other posters here might have but my thoughts are...
... don't sign or commit to anything on paper.
The reason why is that things can change a lot in 6 months. Why tie yourself down? Good decisions should always give you more opportunities... whereas bad decisions take opportunities away.
I'd tell the REA that while you refuse to sign anything, you are happy to let the interested buyer/property developer (sounds like it) know verbally/informally when your house will be put on the market and at what price. They can compete with all the other interested buyers. More competition = higher price.
My thoughts that it will work in your favour in the long-run. All the best
