Rustic Sash

Above are three items I own. Am I correct in saying that the Rustic Sash is the best of the three? Why are the other two yellow, which I'm guesing means they're more valuable (sorry I'm new)?

If you are attempting to maximize physical damage, then the Rustic Sash is indeed the best of the three.

Items (and many other entities in Path of Exile) have 3-4 levels of "rarity-" normal (white), magic (blue), rare (yellow), and unique (orange-brown).

Normal items have consistent base stats or base stat ranges. Weapons have phys damage, armours have their defensive stats, etc. For some items, this includes an "implicit modifier" or "implicit mod" for short. Different belt types have different implicit mods. On the hover-over for a belt, you can see the implicit mod section directly below the level requirement. What I mean by "consistent base stats" is that, for example, every Heavy Belt will roll some +strength within a set range, every Studded Belt will roll some % increased stun duration within a set range, and every Rustic Sash will roll some % increased physical damage within a set range.

Affixes are modifiers to stats that roll on magic, rare, and unique items. The developers have released data on affixes on the Prefix and Suffix pages. Those pages do not specify which affixes can spawn on which items; this is mostly tracked by the community. I use this spreadsheet to look up what items can spawn which affixes.

• Magic items have 1 or 2 affixes - up to one prefix, and up to one suffix.
• Rare items have 3-6 affixes - up to three prefixes, and up to three suffixes.
• Unique items can have any number of affixes, and they are custom-defined to be consistent (like implicit mods). For example, every Shiversting has some %local phys damage rolled in a set range of 80%-100%, and will always have 35-70 cold damage, 3% phys attack mana leech, and Cannot be Frozen.

Whichever items are "best" are dependent on the specifics of what a character needs. If your character needs cold resistance, for example, then the first belt is best. If your character needs more Strength, for example to meet skill gem requirements, then the second belt is best. If your character is killing things too slowly and deals physical damage, then the third belt may be best.

Whichever items are "more valuable" also depend on character specifics, albeit somewhat less so.

• If you're talking about "valuable" in terms of selling to NPCs in return for currency orbs, then yes, rare items are more valuable than magic items are more valuable than normal items.
• When trading to players - the lower level your gear is, the less likely you will be able to sell. Very few people would want to buy your first two belts.
• Items are highly valued by other players when they have both desireable affixes and high-rolled values in those affixes. For example, endgame belts can roll up to 40% on cold resistance, and resistance is a highly desired affix. By contrast, reduced attribute requirements and increased flask mana recovery rate are niche affixes, which are less desireable and you are less likely to find buyers for such items.

Wow, thanks a lot! This was a great help.

No problem! I also have a couple of extra points I need to mention:

• I accidentally said Lioneye's Glare when I meant Shiversting. Oops. Fixed in the post above.
• When selling items to NPC vendors, I recommend IDing yellows but not blues. This will help you save scrolls if you use a lot of scrolls.
• This guy named Noperative wrote a game guide, including descriptions of properties that make an item "good" for selling to other players. Click here to see it. Use Ctrl+F to find "820" if you want to skip to this list.