You should not offer it, if you make the sale do you want to announce it? So just do the deal and announce it, if they ask you for an NDA you can then decide how much you want to charge for the NDA. I want my sales public and believe in price transparency, I would never sign an nda (with the 1 exception that it was life changing money, and I would never have to be involved in domaining again) and have both personally and had clients I was brokering for tell the buyer, no NDA, if you are insistent there is a fee, quite happy continuing to own the name.
The best one was brokering for a client and friend in .tv many years ago, the price was $15,000 and insistent on an NDA. My friend said no to the NDA, .tv sales being announced is important to me and my portfolio, he went higher than Sedo, the price for the NDA was $16,500 so 10%. They said forget it, my client told me to tell them they already forgot their name, and some other stuff I could not mention, it got quite contentious. One year later the buyer came back and paid $16,500 and got their NDA. So they postured for nothing and wasted a year.