Can I use flooring as baseboards?

No, flooring is not finished on the sides.

You can if you want to but baseboard trim would look better. It would depend on how particular you want to be with the job you are doing. in some circumstances using the leftover flooring as baseboard may be OK. It would be hard to make any mitered corners and the top edge of your flooring would be exposed once you have it up on the wall.

Yes you can, but...you will have raw edges if you want that look. I've known people that took trim to cover up the raw edges and one who chalked and grouted around the raw edges and all three ways came out looking good.

Quarter round