Selling a MacBook Pro that functions, but must be plugged in at all times?

I'd sell it but state the issue, someone may want to to use it plugged in, otherwise they may just want to part it out.

what's it take (cost) to get that fixed from an apple store?

Having it plugged in at all times traditionally meant that it is running at half speed. At least that is the way they used to run if there was a problem with the battery.

I'd probably sell as spares or repairs.
Imagine Apple costing more than its worth to the point you will pay apple as much as the laptop is worth to get it fixed.
Someone out there will have a logic board but need a screen, ram, case etc.
I'd just ebay it with no reserve and let it find its way.