Before the DC Upgrade on April 8, 2002, many things in the game were predictable if you happened to be the type of person who had a knack for noticing trends. The in-game value of items was one of the more predictable things. Back then, it was rather easy to predict the value of an item to the in-game shop owner because she was willing to pay the same amount of marks for other items that had absolutely no relation with one another. These items included:
Blast Powders, Mandrake Roots, and Rubies.
Bless Scrolls, Emeralds, Ginseng Roots, and Thief Insurances.
Camping Gears and Healing Salves.
Crystal Crowns and Enchant Scrolls.
Garnets and Gold Apples.
Gold Nuggets and Panic Dusts.
Platinum Rings and Youth Elixirs.
In other words, the in-game shop owner paid the same amount of marks for Platinum Rings and Youth Elixirs; Garnets and Gold Nuggets; Camping Gears and Healing Salves; etc.
The Predictable Junk
Of course, items such as Teeth, Food, and Fish still sold for exactly 1 mark regardless of the hero's stats. And people still couldn't sell Turnips, Troll Warts, Bushido Tokens, Kookies, or Rocks to the in-game shop owner. It's a shame though because the latter two items were sought after by so many traders in the Trading Post forum!
Charm Determined the Price
In general, the value of an item to the in-game shop owner wasn't dependent on the hero's level although I used to think that it was true until one of my lower-level characters was able to sell items to the in-game shop owner at a higher price than my higher-level characters. I had to rule out Guts and Wits because they didn't seem to have an effect on the sale value of an item. Soon after, I discovered that Charm was the sole factor in determining the value of an item, which made perfect sense. The more Charm that a character had, the more marks that the hero was able to divulge from the in-game shop owner. It's too bad that having Charm in real life doesn't give you a discount on merchandise though.
Having the Merchant clan ability (weapons with clan-specific rare attributes didn't exist back then) was also a way to sell items for more marks.