Your best bet is to find someone local who is looking for an engine and knows you to be trustworthy. Faced with a "gently used" ZZ4 on CraigsList for say $3200, or a new one with GM warranty for $4100 I'd likely spend the extra grand to get the warranty. Like Dave S says, to a stranger the used engine is going to be lucky to bring 50% of the lowest advertised price someone can find.
Just curious, but why are you looking to change? I would think your ZZ4 would make the Deuce scoot!