“We need to think about it.”
Why they say it. It usually masks a specific unspoken concern, price, an internal stakeholder, or doubt about value. They want to avoid conflict by deferring rather than naming the real blocker.
Do not: Do not just say sure, take your time and hang up, which leaves the real objection buried and the deal stalled.
Of course, this is a real decision and you should think it through, I would never push you to rush. The only thing I would hate is for you to be weighing it on incomplete information, so let me make sure I gave you everything. When you say think about it, what is the part that is still open, the value, the price, or getting others on board?
Then ask. If you had to point to the one thing that is making this a maybe instead of a yes, what would it be?