When you try to activate the phone, are you also setting it up in your account? The previous owner has to remove it from his/her account first. Also, you can't put your name on it until the previous name is removed.
Go to the technical support area of the tracfone website and see what it says about the phone. Then report back.
What model phone is it? Look at the label under the battery.
That phone is a good one. Don't be concerned that it's old.
The seller needs to remove the phone from his/her account. But that is not enough. Tracfone still has his/her name in the records for that serial number. The account thing is for convenience so you can do things a little easier. But the serial number will still have a name in the records if the seller had any dealings with customer service such as help adding minutes, etc, because they ask for your name. That's how you prove you're the real owner if there's any doubt. Anyway, ask the seller to take care of this. He doesn't have to put your name on it, he just has to get his name off of it.
I emailed the seller but haven't heard back and I ended up contacting Tracfone through Facebook and got some help that way. You're right, millept, they said that they would need to send me a new SIM, which should ship Monday. It sounds like they were able to remove the phone from the seller's account, but we'll see what happens when I get the SIM and try again to activate the phone. Thanks for the help I got here!