I would love to see some conclusive research on this topic. I couldn't find any on these forums (my search-fu is weak and ineffectual). Can anyone point me to it? If there isn't any, perhaps I can help somehow.

Specifically:

If the goal is to raise piety by getting rid of an item, then which method results in greater piety gain:
1. Sacrifice the item
2. Sell the item to a shopkeeper for gold, then sacrifice that gold

Clearly the answer depends very much on the character's ability to get a good price in a shop. So the shopkeeper's racial prejudice is the largest factor, and we need to know the character's race, gender, and charisma / appearance. Potentially the haggling skill would come into play as well.

Simply stated, selling an item to a shop results in the receipt of gold to the tune of X percent of the item's value. Sacrificing an item on an altar results in the receipt of piety to the tune of Y percent of the item's value. X is fairly well known and depends on the previously stated variables. What about Y?

The guidebook says Y is known for gold (.32 / coin, .48 for dwarves), but gives no clue for other items.