There is a large markup on jewelry due to the overhead costs moerchants incur in keeping such expensive, slow-moving inventory. Here are some general guidelines. If trying to sell to an individual, you will be lucky to receive two-third's of your original cost. If selling to a store, you will be lucky to receive one-third of your original cost. If you choose to buy for investment purposes, buy at as close to a wholesale cost as you can get.
Fine jewelry made of solid gold and unusual high-quality gemstones tend to keep their value better.
Unique and collectible items retain their value well also. Colored diamonds have increased in value over the past decade. You make wish to consider resetting stones or recasting metal if you want to update your jewelry collection. In my recommendation, I observed Net-etiquette. Most other jewelry-related newsgroups, do NOT allow for sale ads.