With the broken shaft, you will probably not get much more than $100 - $125. It is a very useful little mixer. Not much power. If I recall correctly it is 250 watts into 2 ohms, so that's about 60 or so into 8 ohms.
I would keep it as a backup or extra mixer for little "gigs" (sorry Uncle Dave).
Generally used gear will bring around 50% of new price, unless it's really close to new. In this case you must deduct for the boo-boo.
DonM
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DonM