You only need 250 scores to get the avatar, unless they have changed that.
To make trades at that game, you need to check the number of red bars the card has. People will only accept if the card you want has the same or less bars that the one you are offering. If you want to trade more than one card, the same rule applies to the sum of the bars the cards have.
For instance, you can change 2 superstar cards (1 bar each) for 1 ace pitcher card (2 bars).
Hope it helps.