We've broken a few of those splined shafts as well. I'm not sure if it was designed that way or just a lucky coincidence, but they seem to be the weak link if you jam the carriage- they break before the more expensive gears. JT has always replaced them for free.
The table rock issue has been addressed here before with some good tips on how to solve it- do a search on this forum.
Before trying to sell the unit, you should at least get it back in working order.
You might consider doing what JT has done with the new VFD machines-
The CNC will perform all the tasks that the gear train does and more, so you could just eliminate all the gears completely.