Don't ever sell games to retailers, they won't give you much. I wouldn't do yard sales if you aren't sure how much the games are worth - you might have some gems among them and not be aware of it. Only do it if you know the worth.
Selling online (amazon's my personal choice) would probably get you the most out of your games. Amazon's really easy to use, and you can also (usually) use it as a reference to current prices. A few months ago I sold 30 or so games and got near $600 total. But it really depends on what you have. Games like the .hack series can sell for a good amount.