Source: Anime News Network
Konami Digital Entertainment has announced on Friday that Guinness World Records has recognized Yu-Gi-Oh! as the best-selling trading card game to date with over 22,587,728,173 cards sold as of April 2009. The game entered the record books on July 7. The official announcement was made at the 7th annual Yu-Gi-Oh! Trading Card Game World Championship in Tokyo. Kazuki Takahashi drew the original manga from 1996 to 2004, and the card game first shipped in 1999 and eventually spread to over 45 countries. Konami had applied for the record back in 2006, when "only" 15.8 billion cards had been sold.
That's alot of cards. I have some old ones lying around I think. From when I bought them for my niece. I wonder if they're worth anything :)