That's a refreshing thing to see too, given the number of folks that seem to base their pricing on Crack-pipe fantasies!
At one time, I had a partially complete SB-9". Attempting to source a tailstock alone, for less than that sale price, proved damn near impossible. I had found a casting, in the raw, for a couple bucks, but the guy that needed it more than I did took that with him when he carried it away (I sold it to him as a project machine, to be clear!)
While it pains me to say so, I feel pretty comfortable saying that parting out some of the major bits would likely provide a pretty decent return on investment, at that price! Chuck, headstock assy, drive unit, apron and carriage, tailstock, the gearbox itself (even if badly built, so long as the gears are not destroyed) and the cabinet itself, all seem to me to have some more value than the $300 asked.