Etopps is a baseball card stock market for people to buy and/or trade cards.
Every week etopps (Topps is the company that runs etopps) will add new cards. For instance this week they have a Jordan Zimmerman rookie. During the World Baseball Classic they had cards totally devoted to it.
When these cards come out you can then purchase them and put them in your portfolio. For instance the Jordan Zimmerman card is $6.50 if you purchase it today. The value of the card will then go up or down based on how it sells on ebay. If people buy it for $2 the value goes down and if they buy it for $20 it goes up.
You can also make offers to the people who have them up for bid. For instance if someone has a Derek Jeter card you want you can offer them 4 cards of what you think may be of equal market value.
Many people never see the cards and just trade them in and out of their portfolios. Others can order them from Topps once they are officially available which is about 6 months time. You will pay shipping.