sold a transmission now having buyer problems

You're not responsible for anything. He needs to confront the transmission shop for not following the proper procedures before installing the transmission. He got a working transmission for $120. You clearly told him what needed to be replaced and that the transmission will move his vehicle.

I paid $75 for a transmission and drove it around with no problems. I had issues with my oiling system leading to turned rod bearings and I removed my engine (left trans in vehicle tied up). I went to the local salvage yard and picked up an engine. I rebuilt it and dropped it back in without thinking about changing the front seal. It leaked immediately along with taking out my front pump bushing. I cannot blame the guy I purchased the transmission from. Once it left out of his possession and it worked as he said, it was a done deal. Anything after that falls back on me.