I don't think the US sold him chemical weapons, maybe the chemicals to make chemical weapons. But Donald Rumsfeld traveled to Iraq to sell him high resolution satellite imagery of norther Iraq, so Saddam knew exactly where the chemical weapons he attacked the Kurds with needed to be dropped.

while the US did a lot of shady things in Iraq, selling chemical weapons was not one of them. Iraq manufactured all of their CW, and the expertise to make those factories came from Germany. The financing to build those factories came from the UK.