Your in luck!
The Fremont has a seafood buffet every Tuesday and Friday featuring hot crab legs(well, warm anyways). I try to go there almost every time I go to vegas. Infact its on my upcoming itinerary for Friday september 21st! (the hell with nutri system--Im in vegas baby!)
It may not be the biggest and best seafood buffet in Las Vegas, but its the only one that "I" know of on Fremont street, and I think the value is there exspecially if you like crab.
Enjoy Weggie