You'd need a 301 redirect - that would send visitors seamlessly to your site, without stopping at the other site at all.
But think carefully; is it your domain they want, or other 'similar' names; in other words, how valuable will those few extra visitors a day be?
Check his site - code view - and see what kinds of key words it is optimised for. It may not be your niche at all, despite the name.
Also remember that the traffic he's getting may be faked; either redirected from elsewhere, or created by him.
What's your site's conversion rate? if it's 2%, then he's sending one paying customer per day. What's your average customer's spend? And how much is profit? And how much will you get, once you paid him his fee?
Personally, I'd multiply the monthly figure he's asking by 12 and offer to buy the domain. But if that sum seemed too high, I'd just walk away.
But be nice. His next trick may be to open a porn site if you decline!
All in all, the chances of this being a good deal for you is very remote indeed.
[edited by: Quadrille at 4:59 pm (utc) on Jan. 6, 2008]