One of the best ways to add to your baseball collection is to purchase the on ebay. Tons of people sell both modern and vintage cards. For a little bit of cash it is possible to add a huge amount of money to your collection. One great thing to do is to purchase bulk cards, and then look through them valuable ones. You can then trade the rest.
If you want to get top dollar prices and cash for your vintage pre war baseball cards call Sean Bassik Cards or Just collect. As sports cards collectors and buyers these appraisers can buy your memorabilia collection or you can sell it via consignment on auction sites like ebay.