noticeable VIBRATION in clutch pedal only

Hey there
I'm looking at a used 2005 Mazda 3 sport 5 speed with 155,000km. The car is in GREAT shape, except for a vibrating clutch pdeal when in use. In neutral, no vibration. But when driving, every time it is used, there is a good vibration in it. I can even feel it if I ever so slightly depress the clutch.

The vibration is not felt anywhere else - just the clutch pedal and it shifts no problem. No noise, nothing.

Is that a sign of trouble looming?

Thanks!
dave

Could be the throwout bearing. Any noise while it's depressed?

does the clutch make a chattering sound when depressed? it is most likely a throw out bearing (owner is trying to get rid of the car before clutch grenades and destroys transmission)

or simply clutch chatter which is nothing serious.