Secret: Time is NEVER on your side when you are buying. Never forget that. Keep in mind that if you are looking to explore the viability of a purchase, a good starting point is the LOI. Once you get beyond the "overview" of what you and the seller are looking for, detail everything out in the purchase contract, and keep all of the above in mind when doing so.
There are enough litigation examples on this issue around to suggest their fears are well placed. A confidentiality agreement (also called a non_disclosure agreement) and followed by a letter of intent is the two step process for allowing a business concept owner to establish a new business relationship without losing control of their business idea.