I'd also check the gun shows, about 5 yrs ago my dad got a police trade in at a gun show in San Antonio for about 200, it was park'ed, had wood furniture and was in excellent condition. the guy that sold it to my dad said that that specific one was one issued to a certain officer, hence the good condition, and the rest were ones used for training puposes and were visibly more beat up.
he took it to a gunsmith in Laredo, TX to get it checked out and everything was in good condition, no parts had to be replaced. it also had an engraving in the reciever with the initials of the p.d. and a stock number. he put about 50 rds through it throu out the years and never had a problem.
he ended up trading it for a mini 14 and sold the mini for 400 bucks last year to finance his first AR.
I on the other hand got a new one with synthetic furniture for about 450 + tax. i didnt want a used one.