Basic ways that you can make some money, by selling Yugioh Cards.
The first step into selling your cards is to make sure that your cards are in mint condition, and in playing card sleeves. I say this for 2 reasons: bad things can happen, and mail travel can wear out a card if unprotected.
Selling the good cards, depends on the market. Usually after a Shonen Jump Tournament, usually duelists would like to buy the cards of the winner's deck. Which means that if you have the cards in that person's deck, you'll make some money.
Also a good step is to check out the yugioh website at: http://entertainment.upperdeck.com/yugioh/en/gameplay/faqs/default.aspx to see the new Forbidden Lists. And be warn, you'll see that forbidden cards reduce the market value of your card significally.
Be creative on what you're selling. One good technique that I like to do is to make creative decks (decks that are not starter's deck). For an example, a Mai Valentine Deck with harpies, harpie's pet dragon, etc. Or even a Kaiba deck with the blue eyes white dragons, blue eyes ultimate dragon, and even obelisk the tormentor. The more creative that you are, the better.
Sometimes you can go to a local card shop, if they sell cards. Most likely they would look for high-end cards, but if you play the right cards (no pun intended), you can sell your good cards.
If all else fails, or you don't want to pay sellers fees & shipping prices on ebay, you can go to this website: http://www.sell2bbnovelties.com/ I've been on the site, and its a good site to sell your cards. All you have to do is look at the price list, and you can list what cards you want to sell and get a good price. Although you'll have to pay for shipping, you can clear out some card space, especially by selling common cards.