How to smooth/flatten a butcher block
Finding a shop with a large enough planer or drum sander would be your best bet.   But I have found when I plane a rough, warped board I end up with a smooth, warped board!  To do it properly, you may need to make a "sled" and support the board so it doesn't rock.   Run it thru the planer or sander and get one side truly flat, then remove it from the sled and plane/sand the other side. Sounds like a lot of work for an $8 garage sale find. Up to you of course. If it's not badly warped, just planing it might get the job done.  Good luck!