why did the head crack? why is it only $300? why did the owner replace all those other parts (there's more than $300 in new parts listed).
Most of the time, a "cracked" head is caused by seriously excessive heat. Like, running the bike with no oil for an extended period of time.
Also, a "cracked" head cannot be fixed normally. It can be (sometimes) but ends up being more expensive than purchasing a new head.
More is wrong with the engine if the head is cracked. It's not an isolated problem in 99% of the cases I've seen.
For $300 it could be a good parts bike (if you need one), but don't expect to fix it and ride it for less than you could buy a good used one!