Agree 100%.
In my considerable experience, when somebody asks you to sign an NDA, or won't give you any information without some arbitrary contract signing, it's dollars to donuts, an idea just like this, "you know existing site that makes billions of dollars, yeah, like that, actually, can you just copy that", and not only that, they are the FRUIT LOOPS, the biggest PITA clients you will ever get, they have no idea what they want, except that it's not what you've shown them, or it was when you showed them the concept, and then wasn't when you did it.
They never have any money. They will want you to work for a share of the non-existant (but sure to be billions of dollars worth, because look at how much those other sites are worth) profits.
If somebody contacts me and they either say:
a) "I need you to sign ......." (with not even some high level overview)
b) "please call me on ........" (with nothing to say what about)
c) "I need some work done on my website" (with no hint of what the website is)
d) "can you add this to my website, but I won't give you any server information, you will have to send all your work to me in a zip file"
I send them to the round filing system in the corner. They all spell TROUBLE.