Internet Auctions like Ebay
To use Ebay you must have a User ID. This is something you pick for yourself when you first register. Since more than 100 million people have registered at Ebay and each ID must be unique, you may have to try a few before you come up with one that isn't already taken.
Experienced Ebayers often have more than one account. Typically a collector would like to sell with an ID which has an established reputation for knowing about the items for sale. However, that same seller would prefer to buy anonymously.
If you collect specific items you will soon recognize the ID's of others who tend to buy the same things. From there it is a small jump to seach for auctions your competitors are bidding on, and outbid them. If you are trying to buy for re-sale this is annoying.