I think you couldn't make much money out of this anyway. 5k daily visitors sounds decent, but they are of low "quality": They quickly leave and would generate bad click-through rates. It would probably yield less than $100 a month.
I don't think it's actually illegal to do that. Just a bit morally off.
They were sent to you, you have a right to sell them (so long as you're in a one-party-consent state when it comes to recording). You're up to snuff law-wise.
Definetly a shady thing to do.