Sounds to me like the previous owner may have changed the speed on "line con 0" as you initially get output but then it stops, although I do have two hangups with this thought. First, you will often get garbage characters instead, but this is not always the case.
Second is in another answer you said you are booting with the config-register of 0x2142, so any config shouldn't be applying. However maybe this is stored outside the normal config and as such still applying? I know things like the boot variables typically do this, but I would have to do some digging to recall everything that is like that.
Did you try other "standard" serial rates other than 9600 to see if it connects? Here is the list that I know of:
You may also want to try playing with the signaling and flow control as that can be changed as well. However I would start with speed, as it is semi-common for people to increase this as people often find 9600 slow. The only reason I have known people to change the other values (or use a slower speed) is to make it more difficult for people to access the console port.