In this context, "in good nick" means "in good condition," but it's also a play on words, in as much as one might put nicks in a new guitar to make it a relic.
"In good nick" is a common phrase in Australian English, and probably also British English. "Nick" has multiple meanings in these varieties of English. It can mean "to steal," as in "someone nicked off with (stole) my bike." I can also mean "to leave," as in "I'm going to nick off (leave) work early." If you look up "nick" in an Australian dictionary you'll see half a page of definitions.