I don't see any examples of a 721 on Ebay, even completed items.
I'd say it depends on the overall condition of the turntable in regards to it's operation. Specifically, will it work in auto mode. Will it auto-cue a record? Will it auto-return? Does it come with the tall spindle? Has it been serviced recently, or sitting in an attic for 20 years? If it's been sitting, the grease is all hardened, and the automatic features simply will not work without a re-lube. Is the pinch-roller worn? Does it turn the platter at the correct speed?
Many old examples look nice, but almost always require a professional lubrication and replacement of a handful of little parts that tend to wear out. If you've never done it, beware, it's not exactly a DIY project for an average joe. The mechanisms are very complex. You fix one thing and break three more.
If the seller can demonstrate that all features are working then DEAL. Otherwise, it'll needs $100 (at minimum) of parts and service. If the deal is local then slightlty better becaus you save on shipping and turntables are delicate.
Good luck!