If you delay in renewing it, they may bombard you with "special offers" on condition you sign up to their auto-renewal-charge-your-credit card scheme.
If you do that, remember to cancel it the week after you renew.
I have often bought retailer surplus stock on ebay or reduced from a retailer. Bit risky as a crook might sell you a counterfeit licence number.
in Germany (possibly elsewhere) the law does not permit them to prevent a customer selling on bundled and unused software, for example if you buy a PC with Windows and Norton included.
Last year's version is fine because you get automatic update as soon as you install it. I have done that a number of times.