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 100-150mm 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.
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.