fanboy2000
Adventurer
Gems, IIRC, sell at full value in 4e. In the real world, commodities also sell at more consistent values.Considering I was in the business of buying, making, selling, and faceting gemstones and jewelry for over 10 years its also one of the most knowledgeably said funny thing you have read all day.
I don't think that magic items and NPC equipment trade in the same way gems and precious metals do. I think they trade more like automobiles and comic books. I've had sell a lot of items recently and 20% sounds about right on average. I bought a couch 2nd hand off of Craig's List from the original owner for $100. My girlfriend and I estimate that the couch probably cost between $500-$1,000. So somewhere between 20%-10% of the original purchase price.
This reminds me about a price guide I was reading. (Not a comic book price guide though) that said that the prices listed were what a person looking for those items can expect to pay. If someone wanted to sell the item, then the collector should expect to get less.
A business has more inventory and more resources to sell items for a higher amount. An individual does not. Often, an individual has only a short time to sell it and can't afford to turn down an offer. If the individual is selling to a business, then things are worse because the business has to turn a profit. Like RC said, the closer the seller gets to the end owner, the more money the seller will make.