I don't think Buffalo needs Koskinen but I'd do that trade as Buffalo.

Could make a good starter or a good backup

I don't think he's a starter but if you can pick up an asset and have him as a backup/1B then there's no harm in doing so as the Sabres. They aren't competitive anyways and have the cap space.

I definitely wouldn't take him for free though.

The thing that's missing here is how you'll use that 4.5M in cap space to get a new goalie.