And the seller sold the camera as mint? Was the camera guaranteed??? One thing that must be remembered is that there is the "normal way" and then there is the "hasselblad way". For example, camera must be cocked before removing the lens because the lens can only be removed or mounted on the body if the lens is cocked, and to cock the shutter in the lens, you have to do it via cocking mechanism in the body... that little circular pin at the bottom of the lens mounting ring of the body...... it has to align with the lens pin so that they can lock together). If the body isn't cocked, they won't align. It's the Hasselblad Way, or you have a broken 120 Planar like I do. Again, as Mike mentioned, the problem could be the film back.
For starters, if the camera is cocked, (cock it if it isn't, ) and then remove the cocked lens... one less thing to worry about. but from what you describe, you can't advance the film, which means that you can't cock the lens and remove it. A possible culprit could be the mechanisms relating to the film spindles in the film back.... film jamming, the film counter jamming......
Next thing is the remove the film insert., remove the film and put the insert back in. That should reset your film counter to 0. . Now remove the film slide and try winding now with an empty film back. Also, if you want to remove the film back, the slide must be in... if the slide is out, then the button you flick will not budge.
Let us know if you find a solution. Good Luck!