Cisco's set up such that you really have to buy smartnet. You're in for a world of hurt if you try not to. No updates for you, except for critical security updates if you're very persistent with TAC.
Cisco's official party line is that software licenses are not tranferrable. That is, if you buy a Cisco device from someone other than an authorized Cisco reseller, you do NOT have a license to use IOS (et al.). You therefore are not eligible to add smartnet to that device, at all. You must first pay to relicense the software, which, conveniently enough, costs as much as replacing the box.
That said, if you have a friendly Cisco sales person (as in, real Cisco employee, sales guy) and ask nicely, they can often wave the whole relicensing nonsense if you're buying smartnet and promise not to buy used Cisco ever again.