He is legitimately strong, but the World Martial Arts tournament draws legitimately strong people out of the woodwork all the time. It was treated as a joke in Z, but in the original Dragon Ball there were serious contenders that would show up for the prize money.

He just has fixers to make sure he stays on top. He's not getting any younger after all!

Also I think Buu does this scam with him too now that I think about it (mostly for fun, it's not like Buu needs money)