While all items everywhere have some value to someone, when you’re ruling out value of the sentimental variety, there are some definite rules you can look to to make sure you’re always getting the best deal on an item.
1) Precious Metals or Other Rare Material
Particularly with jewelry or antiques, it’s common to see pieces that look ornate and expensive, but upon closer inspection are recreations or facsimiles. Gold, silver, and platinum are non-ferrous metals, and the pure forms of these do not react to magnetism. A simple wave of a magnet over an item should inform you as to the amount of iron contained within the item–if the item reacts at all to the magnet, the metals are not pure or real.
With stones themselves, make sure they’re not glued into the item at all. Stones handled by a real jeweller will be set into the piece, as not only does this look better overall, but it rightly puts the jewel at the center of it all.
2) Is it “Complete” or “Near Mint”?
When an item is brand new, it generally comes with packaging, manuals, and sometimes other extras. For example, video game consoles would typically have AV cables, a controller, and a power supply. Selling the item without these pack-ins is most certainly possible, but any item that is in a complete state (includes all pack-ins expected) will always sell for more. Make sure that anything you purchase is as close to complete as possible. A cursory internet search should be able to tell you what that means for your item.
Much like completeness, the closer a used item is to “mint” the better. Items considered mint ordinarily cost a lot, but if they’re taken care of well, they will last a lot longer than an item in merely good condition.
3) Shop Around and Compare
While the above two methods are useful for determining the value of those things that have a very even set of standards applied to them, it should also be noted that not everything can fit into those lines of inquiry. There are many items for sale whose value is determined almost solely by demand. To find this, make sure to check every point of comparison you can–the Internet (particularly sites like Reddit), friends, family, and other pawn shops–for clues as to just how much your unique item might be worth.