You can post some image of it, there's no harm of showing the interface / theme of your website.
Not disclosing the most important ones will make you struggle to find a buyer.
Information like the amount of visitors it gets, the ranking on Alexa, its hosting service expiration, the high quality existent and good seo he is making reference to.
I agree even these information will suffice the question of an interested buyer. Giving these few info will make them loose their doubt if you are transparent.