I see a lot of sports cards sold on e-bay, so I thought it would be nice to help out the beginning collector with some advise from someone that has been collecting for 25 years.

Stay away from baseball cards produced between 1986 and 1994
   With very few exceptions, these cards were produced in such quantity, that the demand would have a tough time surpassing the supply.  For example, a 1989 Donruss Curt Schilling rookie may seem like a nice investment at $.50, but these cards are far too available for them to EVER move up past $2.00. 

Stay away from nonrookie, new issues
  This may seem like bad advice since these cards trade so heavily, but the next season, these cards lose their steam and the demand drops when collectors go after the new issues.  Save yourself some money in the long run and target cards that are at least one year old.  For example, I'm sure that you would like a ton of Ben Roethlisberger's 2005 cards, but if you wait for a year, you can get them at half the price.

Avoid the overhyped rookies
  If you try and get all of the rookies of the guys that are supposed to be "the next big thing", you will end up with one or two nice cards and a pile of commons.  If you spent $50.00 on rookies of Cedric Benson, Alex Smith, Carnel WIlliams and Mike WIlliams, you would have about $10 worth of cards now.  Wait for the hype to die down on rookies and then you can focus your money on the guys that actaully pan out.  Save your money by going after more rookies of the more established players and you cards will hold their value better.

Buy multiple cards from the same place on e-bay.
  If you buy cards from 20 different places on e-bay, most of your money will go towards shipping.  For example, if you picked up 20 cards from 20 sellers, you might pay $40-$70 in shipping.  If you pick an e-bay store that has a lot of what you like, you can pay a little more for each card and save the money on shipping.

Try etopps.com or thePitt.com to save on storage
  If you're like me, your collections start to take up more space than you have and you can start to lose track of what you own.  By using etopps or thepit, they hold you cards which saves you the cost of shipping as well as holding your cards for you so they are organized and won't be damaged.

Have fun with it
  Try and collect players / teams that you follow.  For example, try and put together as many Steeler rookies as you can or as many Barry Bonds cards as you can.  This gives your collection more of a personalized theme and allows you to focus you money on cards that will add to the depth of your collection.