The advice that you "can't sell it" is misleading.
You in fact can sell it, and a new owner can use it in commerce; as long as it does not attempt to deal with commerce related to the core business of WhatsApp. But that is pushing the envelope when it comes to IP/TM issues.
At the center of this is your 'good will'. Which means that you did not register/own the domain to benefit off of the name of WhatsApp. If you sell the domain to another party, that good will extends to the new party.
This does not mean that there is risk involved with the sale/ownership, as I can almost guarantee you that there will be.
It's a massive risk, to own for any new venture.
At the end of the day,
how much are you looking to sell this domain for? Keep in mind that "WhatsApp"
did not pay much for their domain; and likely got it for an embarrassingly low price.
I don't think they are heavily invested in controlling domains, since it's mainly an application and not a website. As on person has said; "I have used Whatsapp for the longest time, I never even knew they had a website, or would ever care to visit it."
Either way, good luck to you.