Need some replacement issues to look for in a second hand stereo

[Equinoxlover]

Okay I need to know if I buy a used stereo to replace my present stereo. Can this be done like I buy an EBay unit what do I need to install one. Or if I got to a wrecking yard, can I just buy one install it and have it work. Or should I buy a new replacement at 700 dollars

 

[rawhite318]

more info is needed? What vehicle is this for? Do you know your radio code so forth and so on. I do know there are cheaper than $700 to change your radio but that depends on the year.