A fast talker
Definition 1 of 2
Pro Tip 1/3Deceptive Intent
A 'fast talker' often uses speed to confuse and convince you of something not entirely true. 
The salesman was a fast talker and before I knew it, I'd signed a contract.
Pro Tip 2/3Stay Alert
If you know someone is a 'fast talker', stay vigilant and question their statements before agreeing. 
He's a fast talker, so double-check the details he gives you.
Pro Tip 3/3Colloquial Warning
Labeling someone a 'fast talker' often serves as a warning about their persuasive but possibly insincere speech. 
Don't trust him; he’s a fast talker and rarely delivers on his promises.