I can’t imagine not tipping at a buffet, the workers come to the table even more than a sit down meal.I never tip on a credit card. We always have cash to tip the server. My standard is 20% if the service was good. If the restaurant employees tip share, the split of the cash tip is on them to be honest with the runners, bus people and kitchen staff.
An area where I always tip that I personally haven't seen others do (I'm sure it happens but I rarely see cash on the table) is at a buffet. The people who regularly keep the tables clear really hustle and deserve something extra.