One strategy, more forward-looking, is to set up a saved search with an online marketplace like eBay.
With a saved search, eBay would email you when an item matching your search gets listed. You can add the items to your personal watch list and monitor closing prices or failed listing prices.
Often, it's buyers that use the feature to locate newly-listed items they're interested in bidding on. Yet, using a saved search is also a valid strategy for a potential seller looking to find out (a) the likely value for an item you may want to sell, as well as (b) what kinds of listings work best (e.g. free shipping, or not? auction, or set price? How should the item be described, if you do choose to list yours? etc.)
Over time, assuming the item you're interested in selling has an adequate market, you should accumulate sufficient information on the price for similar items to inform your decision to keep or sell.