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.
Few details would do but there are details no matter how small or minor it is, it would already disclose out sites informations or stats. For example how you would able to give out alexa ranking? For sure you would need to input details about the site, another example if you do make or picture out a glimpse of the site and giving out some image or even keywords it will definitely give you a hint on what kind of site are you looking at thats why i do prefer disclosing it on the very first place,i dont know what would be the doubts.