Question about economics

[Neubert]

(For the record, I am not actually sure what an armbow is.)

If the goblin knows or learns of the true value of the item (he might think it is a mundane and/or cheap item, considering the way it was traded).
Assuming he learns of the true value, and he somehow sells it off for a big sum of money, there can be a few possibilities in regards to effect on economy (he could pass it on cheaply and someone else might figure out it's worth, but let's not make it more complex than it already is).
Either he won't want to return, as he is now a wealthy man or he does - the difference is only where the effect happens. Suddenly introducing a character for lots of money  (like a lottery-winner) can mean a few things, depending on the personality of the person. Some will just save up the money (which wouldn't create much effect), but others will use them right away, and possibly get in trouble as they get used to another way of living and then the money is gone. Assuming the goblin returns to the market and intends to use the money means that prices on everything is going to go up. He might buy up a lot of the food, which means the price will go up and others will starve. Since it is more expensive to eat, other merchants will have to raise their prices to keep food on the table. The whole economy on that market could collapse, as costumers would go somewhere else for their shopping (if possible) where the prices have not been changed (a higher demand there would mean the prices would go up again though). The goblin would also be able to buy out all competitors for his wares, which means that he can raise his prices (depending on the product he sells of course).

This is obviously just one direction the effect might take, and it is dependant upon a few assumptions.