Most shops and businesses are open 9am to 5pm, Monday to Friday, as a minimum. Retail stores and shopping malls will usually open late nights on Thursdays and/or Fridays in the main centres throughout NZ. Most retail stores also open Saturdays and Sundays.
Most banks are closed at weekends unless they are situated in a retail mall.
For purchasing of 2nd hand goods of ANY sort New Zealand has Trade Me. Trade me is the largest auction site in New Zealand. You can buy anything from small household goods & furniture, a car, a pet, and even a Home. There is a job search part to this site as well as listings for domestic services which individuals and companies can offer someone.
A bidding system works for most saleable items and specifications re picking up, delivery options etc. are outlined on the sale information.