I recommend buying a new one also, from an authorized retailer. They update these chips at least each year as land ownership, boundaries, etc can and will change.
You will need to register your chip to be allowed to download the updates. If you buy a second hand chip, I don't think the update will download to it since you are not the original owner. You could be relying on old map data that is not up to date, which could result in a trespassing violation. These chips are money well spent, and the update is only $30 per year.
Good Luck