Each item is valued differently in different guilds. The value is generally based on the supply/demand at the time. So there isn't going to be a single sheet you can reference. You need to look at the DKP values for the specific guild you are in.
If you are in the biggest raiding guild, you could probably get an Amulet of Necropotence for cheaper than if you were in a smaller guild. In a smaller guild, that item is actually a pretty big piece because their range of killable targets is more limited.
Also, you probably aren't going to be getting the best of the best loot to sell, simply due to demand. For example, you probably aren't going to get Flayed Barbarian Skin Leggings. Not only are those expensive DKP wise, but many people bidding on them want to use them. So you will probably be spending more DKP on it than average. The guild may also prevent you from selling loot that good, because it incentivizes people to bid to sell, rather than to use.
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