Whether it is true or not raises the question; would it be legal for Trump to pay cash for an endorsement, say, from Sarah Palin or Sheriff Joe Arpaio? Since he is self-funded, I think the laws governing campaign financing and spending largely do not apply. He can spend his own money as he sees fit.
So can he buy endorsements, for cash or patronage?
Sure, why not? I’d rather see politicians buying the endorsers instead of the other way round it’s usually done. Also, Steve King is not a credible source, and his statement is pure fiction.
What legally is an endorsement? It has no tangible value, even in the sense that a politician’s vote does. It can’t be illegal to pay for an opinion. I’d say a court would find he has a First Amendment right to do so.
I don’t believe anybody does pay money for endorsements, but that’s because they don’t have enough value to be worth money, not because it’s illegal to do so.