When evaluating a JY T-case, look for fluid. Is there any? Is it burnt (brown instead of red)? Take out the drain plug and feel for the magnet in its slot right inside the drain hole. Is the magnet loaded with metal particles?
None of this means the case can't be rebuilt, but does indicate chain wear (and perhaps other damage, such as a badly worn mode fork) as well as lack of maintenance, all of which should be taken into account when dickering over a price.