fascinating to read about different experiences on here. as others have said get independent legal advice about this before proceeding. would also get your property valued to be clear about its value. then you can make informed decisions about selling on your terms.
personally can see little benefit to restricting offers to this 1 buyer, unless they will give you a premium price for the exclusive right to buy.
it could work in your favour but protect yourself with all the facts first and do have legal advice. good luck. wonder how common this cold calling with promises is?
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