I live in a small rural town, and TradeMe has provided me with a convenient way to buy everything available to a big city resident from buttons to iPhones, wigs to weedkiller, without any problems. I only buy from traders with high ratings from lots of previous trades and always get great service from these traders. TradeMe's system of showing ratings of the trustworthiness of other traders stops any cheating.
A scammer got my Trademe details once and started to use my account to sell fake goods, but TradeMe's fraud team stopped it immediately.
I think the one star comments on here are all by one grouchy person who failed to follow the above precautions, and is posting the same complaint under several aliases. I have never heard anyone making these complaints. I think most people find TM pretty reliable and convenient.