The seller states that the trans has been totally repaired and filled with oil
I know. I'm just skeptical that if the noise is a sharp loud squeal (which is common when this trans is low\out of fluid) I would suspect low on fluid. You don't don't get a sharp squeal noise from parts submerged in oil(very muffled if any).
My Deere has the exact same trans. I found out on mine, the plug on top where the gear and hydraulics sections connect, there is a filter(i think that is what it is supposed to be) oil would barely flow through this into the hydraulic side.
I drilled and tapped drain plugs on mine. When mine wouldn't go, upon draining it, not much came out the hydraulic side. I THOUGHT i poured in 20oz(or whatever it was supposed to be) but i learned i didn't when i drained it to open it again when mine barely moved.
If the person who serviced it just pulled the plug out and poured fluid in until it was "full" and called it a day, maybe this one was like mine(it is the same model as min but don't know if the filter did the same as mine)- no fluid went to the hydraulic side.
My guess..