We use a postal scale for most things 2 lb and under, and a freight scale for larger items. We picked up both at thrift stores. Freight scales are not something we often see at thrift stores, so you may want to explore other avenues for one of those. We sell a variety of weights and sizes of items. If you plan on selling many of the items that we are, this is essential.
Freight scales for sale on eBay ($150 or less*)
Unless you plan on shipping massive items, you don't need a scale that can weigh in the thousands. We prefer the type with a corded digital readout for easy visibility for odd-shaped, large items. *Note that the feed may provide results exceeding this price.