Like paying his mortgage. Like paying for his wife's car.
The thing is, it wasn't illegal at the time. But it was so disconcerting that (a) it cost him the election in 1988, and (b) the FEC wrote new rules against such commingling of funds. Pence's response at the time was to the effect that a guy running for office has to make a living. You can fill in the details with this Washington Post report.
John Cassidy at the New Yorker lists seven reasons why Trump's pick of Pence made sense. My fave is #7: "There may not be many liberal and moderate voters left for Trump to alienate." And I will add #8. The codes of ethics of these two TP-ers are perfectly aligned. Trump had no problem stiffing his contractors and enriching himself from shady casino deals. Why should he think badly of Pence for pocketing campaign contributions? Those with better resources than I, in this campaign, should follow the money.