Ford Taurus slow turning over

Just sold my '99 Ford Taurus V6 DOHC Battle Wagon to my Cousin. Let him take it for a spin, came back and exchanged title for $. Well it must be mad at me for selling it because when he goes to leave it will not start. It is barely turning over so I started with the battery. Battery is good so I then cleaned all connections to the starter with no change. Still turning over but only barely. Took the starter off and had it tested at O’Reilly’s and the starter is also good according to them. I guess my next step would be to start replacing the wiring from battery to starter? After that I am not too sure on a direction to go. Any advice or suggestions would be greatly appreciated. On a side note, my cousin did not think it was very funny when I told him that the title was already signed and that it was his problem now.
lol.

Sounds like voltage drop. Start by feeling for warm connections then use vom to check for voltage drop between connections. Most likely bad wire between battery and starter.

How was the battery tested?

With starter motors, I prefer to test them on the car and check the current draw as they physically rotate the engine over.

Sounds like a bad starter to me. If the guy at O’Reilly just hooked it up and said “Yup, it spins.”, that’s not much of a test. It will not show how the starter does under load or how much current it’s drawing when trying to turn over an engine.

You might want to remove all drive belts and see if the starter turns the engine over normally. If it does not then it’s either a bad starter or something binding/dragging inside the engine. It really sounds like a dragging starter (very common when worn out).