I agree don't give them enough technical details to take a copy of it with them, give to another code writer and reproduce. Ideally you would want a secrecy or non compete agreement but they would run it through so many lawyers that the agreement will mean nothing or you will die of old age.
Put the words confidential, secret or something on each slide or any handouts then get their names, affiliations, etc. in like a sign in sheet. They know you could use this to prove they stole it if they do something to circumvent your work.
At least put a block diagram or something to show them a black box concept.
Show them how it can be integrated into the existing hardware, if it will, or how it could be used with the objective of a commercial advantage for the seller.
Describe how difficult it would be to recreate. That's certainly part of the value because once someone knows how to do it they can either buy it or develop it themselves. The difference in the cost of these options is part of the value.