The flood of chinese dirt bikes on the australian market makes for an appealing cheap buy but beware - There's a reason they are so cheap!
In comparison to Japanese made bikes the build quality and engeneering design is much less advanced.
When buying secondhand, A thorough inspection is required. - This applies equally to any bike
The following points are suggested but in no way exhaustive;
Wheels - Check for buckles or dents. look for missing/damaged spokes. check bearings by trying to wobble the wheel and look for play
Frame - These are succeptible to cracks, especially around the rear upper suspension mounts. One giveaway is if the stand seems too tall, like the bike is ready to fall over the upper mount may be bent which reduces the bike height and then the stand seems too long. You may need to remove plastic covers, seat ect to thoroughly inspect but it is worth the hassle. Basically try to look at every welded join.
Chain/Sprocket - Look for wear specifically on the sprocket teeth - They are normally symetrical so if they appear to lean toward the front of the bike on the top of the rear sprocket they are due for replacement.
Engine - There are too many things to list specifically but generally feel the compression through the kick starter and listen for rattles when it is running. There should be no oil leaks nor excessive smoke although two strokes will smoke a little which is normal. check the engine idles OK when warm (they always idle rough when cold) and revs freely and without hesitation.
Suspension - Check for cracks previously mentioned. Compress the front suspension and then wipe the chrome tube with your finger - There should not be a noticable oil film on the chrome - If there is, the fork seals are shot. Do the same for the rear shock if possible.
General - check cables are operating freely without binding and that the throttle snaps back when released. check splines on kickstart and gearshift shafts are in good condition and not loose or worn - These will be expensive to replace. Check plastics are mounted properly and no mounts are broken - They are usually mounted fairly flimsily. Check handlebars are straight and not bent
Hope this helps you in your purchase.
Always remember, you are spending $500 instead of $5000 for a similar product so keep in mind you get what you pay for. These bikes are not designed to cope with jumping so no Crusty Demons of Dirt impersonations!