Used stuff is fine if you know the owner and they are the FIRST and ONLY owner. If it's been passed through a few peeps I'd worry a bit. It might be hot and the amp may have been abused. The subs may have been pushed the limit as well. The nice thing about buying stuff new is the warranty. Amps go and subs blow so it's gambling your money. $400 may be cheaper then $7 or 800 but it's still a lot of money.

Personally I purchase all my stuff used except for head units and speakers. But I only buy from friends so I know I can smack em if something go's