its a pretty complete system with amplifier:
4runner - OEM Audio Plus
buying someone's used JBL system doesnt guarantee a complete kit w/ all wiring harnesses and etc etc.. i'd imagine it to be a complete nightmare to install.. plus it'll cost more than OEM plus (not saying theyre inexpensive by any means) but the jbl will definitely cost more.. plus the headache of retrofitting the JBL head unit.. its simply not worth it
OEM plus comes w/ complete instructions and a customer service line as well...