The creatures only have a vague idea as to their value and barter the items (coins) just as they might a slab of meat or hide from the last deer, beaver, wolf, elf, etc they killed. They don't have or recognize 'prices' such as '15 gp for this item, 10 sp for that one'. Instead they will haggle (poorly) and trade pretty shiny coins for more useful items - such as a iron spike they can put through a club to make it a better club for harming or perhaps enough fruit, grain, or even meat (if hunting has been poor) to see them through the next few days or weeks.
They have no idea how much the coins are actually worth, however, and so their 'bargains' tend to be rather one-sided: either the wily shop keeper overcharging or the intimidating creature forcing a frightened shopkeeper to accept what it thinks the items are worth - irregardless of how much they actually cost (often at a notable loss for the shopkeeper).
Most towns do not allow such creatures inside their walls, however, so it is usually the rural village shops that deal with such creatures - and then often only duress or to make a quick profit. Such dealings are viewed as bad luck by most, however, as the creature might wise up one day and come seeking revenge for prior poor bargains. To prevent such creatures from entering settlements, in fact, is one of the primary reasons for raising walls (well, wooden palisades, really) around them - especially in areas that are not often beset by battles. Keeping dangerous creatures in general out of the settlement might be the stated reason, but monsters that can reason enough to want to trade are often considered more dangerous in some areas.
This is not even taking into account the fact that coins from such creatures are viewed as blood money, as they most certainly have been taken from travelers or those recently slain (by wild animals, by the creature itself, by hazardous weather, etc). Superstitions abound about the dangers and possible curses that might be tied to blood money - another reason doing business with such creatures is considered bad luck.