It's sort of a misconception that waitstaff at buffets don't do anything for you. Think about it.
In a regular restaurant, I try to tip around %20, if the service was good. In a buffet it does depend a little, but I try to tip at least %15. Yes, you do fill your own plate - but that is ALL you do for yourself (again, this does vary, at some places you do get your own drinks). The waiter still has to take your drink order, prepare your drinks (i.e. filling sodas) , bring your drinks, clear your plates every time you go for another serving, bring more drinks/water, replace silverware/napkins, prep your bill, bring your CC back to you. It's almost about the same amount of work for them - sometimes more if you go up many times.