NOTE: Average paid price is the price that the
user must sell the cards for to break even. If the user has never sold a card, it is
the average price paid. If the user has sold and bought cards, it is the total
amount spent divided by the number of cards remaining. For example, if a user buys
4 cards at $10 each ($40), then sells 2 of them for $8 each ($16), they have
still spent $24 and have two cards remaining, so their average paid will be $12.
Using this method it is possible to have a negative average price paid. Trades are
considered in the quantity, but not in the average price paid.