Market value is what the item would fetch on the open market. Since the GRG were willing to pay above what the OP was willing to pay, GRG's offer was market value, and what the OP paid was what he thought was fair for the game. Which I think the OP paid a totally fair price.

It doesn't matter what the situation is, if a deal is struck and you give someone your word, you should stick to it. There are always exceptions, but this is the rule that I live by.

I didn't say it was okay to rip someone off, just made the point that I am a man of my word, and I expect everyone I encounter to be as well. It is only common courtesy.

You are confused. What you did was the definition of paying market value for the game. Market value is defined as the amount something can be sold for on any given market. There is no setting of market, there is only a buyer and a seller. The buyer always pays market price, the seller always gets market price. You can do research and come up with a range that you think is fair to pay for that game, but the market value of that game is not determined until the money changes hands.

In your situation the market was you and however many other potential buyers that were interested in that game. You paid market price and got the game. The market dictated that in order for you to get that game you needed to pay a certain price and you paid it. The fact that historical sale prices for that game and other people's opinions of the price you paid says you overpaid, it doesn't change the fact that you paid market value for that game.