Hi

i replaced my 99 4x 4L trans with 180k and it’s sitting in the bed of my truck leaking.

are they worth anything as core?

Would you store it until the current one goes bad again?

If I rebuilt, would anyone buy it from an amateur? (I would be truthful). Trying to recover some costs... I think parts will be $500 and that’s maybe what I’d get.

 

You wouldn't recover any costs breaking even.
Walk into the transmission shops where you live and ask. You can get an idea for demand for your unit.
Contact large transmission rebuilders and see what they say.
I bet established rebuilders have suppliers and wouldn't need to "special handle" your core but you never know.