Buying a second-hand mountain bike should save you a wad of cash over a new one- but just like when scouting for a used car you'll want to keep your wits about you so you don't end up with an expensive lemon.
Bikes depreciate fast through general wear n' tear and rapidly aging technology. Today's 100-125mm shocks for instance blow away the 80mm standard of only a year or two ago. The current strength of the kiwi dollar combined with the recent trend of large retailers direct-importing reputable brands has seen new bikes plummet in price. NZ$1500 buys a quality entry-level conveyance and NZ$3500 lands you a top-of-the-line XT equipped beast. So a second-hand bike needs to be pretty cheap to be good value. As a rough guideline, for a mere 2-3 year old bike expect to pay no more than half the price of the new equivalent.
The classified ads in the Saturday newspaper over a jolt of coffee is a convenient place to start but you'll almost certainly find a better selection on Trade Me or Ebay. Other sources include Vorb, Sportzhub and Mountain Bike NZ. Some bike shops deal in used bikes too - get on the phone to suss out which stock a useful selection. You'll pay a bit more but in return they should have carried out the basic maintenance to bring the bike up to scratch. If you really know your stuff then suss out a Police Auction for un-claimed stolen bikes.
Common sense prevails - a bike that's been thrashed off-road is likely to be overdue for costly repairs. Older bikes may suffer from aging technology. Not only does this mean reduced performance compared to the new version, but in its extreme can cause grief when tracking down compatible parts. A highly spec'd bike that's been confined to gentle rides on the tarmac is indeed a rare and worthy find. Once you've located a bike with promise, run it through the following checklist. Frame. Has it been crashed or run-over? Check the alignment, and for cracks in paint around the bottom bracket and head-tube. Hubs. Wriggle each wheel sideways to test the health of the hubs. If loose, it may just be that the cones need adjusting. However it's more likely that the bearing surfaces are trashed, which means new hubs, spokes and wheel build. Ouch. Rims. Spin each wheel to check it ain't buckled. A slight side-to-side wobble is an easy fix but a flat spot is considerably more challenging and will probably cost a new rim. Drive train. Place the chain onto the big chain-ring then pull it away from the chain-ring. If it's loose by a link or more then the chain is well overdue for replacing. That will involve a rear cluster and middle chain-ring too. Big bucks. Front shocks. Do they feel plush and smooth when you compress them? Are you able to achieve full travel? Beware of nicks or scratches on the inner legs - they indicate excessive wear and can lead to oil/air leaks. Air shocks generally come with a pump, remember to negotiate it as part of the purchase. Clipless pedals... require cleats but may not be offered with the bike. That's no big deal but a new set costs NZ$50-80. And you'll need specific cycle shoes too.
A bike's serial number usually lurks under the bottom bracket. If it's been attacked with a file then beat a tactful retreat. Likewise bikes sporting a fresh paint job and an absence of manufacturers' stickers are dodgy. If in doubt, take down the serial number and log on to the NZ MTB Web Bike Registry, or get on the blower to your local Police Station.
Commence your relationship with your new bike by treating it to a good once-over before your first ride. Clean or replace brake and gear cables and housing. Degrease the chain, cluster and chain-rings. Re-lube the chain and cables. Check for loose parts or those likely to fail. For detailed instructions zip along to Bike Grooming.