If he has the kids almost half time, I don't know why you think he should be paying much support at all.
It may seem petty, but I don't let my daughter go to her mom's house in any clothes I would care about not getting back. She has to change to something older, or something that won't fit her much longer anyway, or something that came crom her mom's house and needs to go back there anyway. I never let her wear her "good" things there. They would never come back.
Dad should be supplying clothing for the kids while they are at his house.
Had the kids outgrown the clothes that he was selling? If not, why would he sell them?