What will happen to all the weapons being sent to Ukraine after the war?

The weapons being sent? Probably either returned or kept by their military.

If you’re worried about black markets, I’d worry more about the truckloads of AKs they handed out at random when the Russians first started towards Kyiv.

There’s usually some small arms black market activity after wars in Eastern Europe, but I don’t think we’re gonna see gangsters with HIMARS trucks.