On SunnyShadows, we offer the vast majority of items obtainable in Minecraft through our in-game shops in Spawn. This means that you can buy both craftable items and ingredients. The idea of the system is to award players for putting in some effort and selling crafted materials instead of raw ingredients. This works the other way around, too; it is more profitable to buy ingredients and craft what you want to make than to buy the crafted item directly!
Not only that: if an item is not stackable up to 64, the item automatically has a higher sell value depending on how many items can exist in a stack. For this reason, selling say, armour, can be quite profitable!
We determine the buy price and sell value for each item in a way that takes into account the rarity and accessibility of the item. Items that are limited in the game world (or are difficult to find) have the highest sell value. Items that are unlimited in the game world, or very easy to find in the correct biome, are less valued, while farmable items are least valued (most ingredients belong to this group).
This evaluation of rarity is not perfect, and thus we will make changes to certain ingredients (which will affect every craft-able that requires that ingredient) over time. This depends mostly on the popularity of the item and significant changes in a Minecraft update, but also just on the fact that when reading through our item prices, we occasionally find an outlier. To make correct adjustments, we are continuously gathering statistics from our shops.