I m not sure about a buffet, but there is a great seafood restaurant in Leawood called Bristol. They have fresh seafood shipped in daily from the coast and it is well prepared. People from the East Coast rave about it as the best seafood in the Midwest.
I m sure there are some places, but I m not a huge seafood fan. Also, you probably aren t going to find the greatest actual seafood in KC since it has to be transported from a sea somewhere. It isn t likely to be the freshest.