Trade-ins: common for cars, less often seen for technology. Right now Harvey Norman is offering $100 off new tablets or computers if you bring in your old machine for trade-in.
The Harvey Norman deal is running until February 18, and only applies to computers or tablets which cost more than $499 and which are purchased in store or ordered online for in-store pick-up. The old computer will be sent to an e-waste recycling facility, which if nothing else saves trying to locate one yourself.
Clearly, this deal makes more sense if you're trading in a non-functional machine; if your device still works, then you may be able to on-sell it for more than $100 anyway. It's also worth bearing in mind that Harvey Norman is often an expensive choice and that its customer service is rarely impressive. The choice is yours.