Wise decision not to repair. The whole machine wore out over time....just the hydros beat the rest of it to the punch....just a waiting game .....what's next to fail.
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I never bought a used mower and never will. Too easy to be abused. One of my ZTs is a Hustler Fastrak 44 I bought back in 2005 and has served me well so I'm not knocking them.
I just bought a new Ferris IS700Z and in doing so I shopped most all other brands. Lot of good machines out there and Gravely seems to be at the top with several others. While shopping $12-15k machines were everywhere so the $12k new price is probably correct for the used Gravely you are looking at. My machine was $7200 new and I bought it for a soft ride and added another several hundred for aftermarket extras to suit me, plastic canopy, balloon traction rears, and extra seat springs although mine has the deluxe seat.
I think an upper cut (not big box store lower cut model built to meet pricing demands), heavy duty, commercial, Industrial Service type machine may be worth the gamble. It all depends on the PO and how the machine was cared for. The reason for selling, if truthful, is a good indicator as to what to expect.