When purchasing beanies, be sure to read the description carefully (especially if there isn't any picture).  If you believe you are purchasing a beanie, you may in fact be purchasing a buddy or teenie.  Look for words such as "new in package," "NIB" (new in box), or "original packaging" (to signify teenies).  Basically, any teenie will be referred to as small or reference being in a bag or from McDonalds.  Also, look for words such as "buddy" or "buddie" to signify a buddy.  There may also be words such as big or large thrown in there as well.

Pictures are helpful tools to help determine what you are purchasing.  Teenies hang tags are big in proportion to its head.  Beanie hang tags are in proportion to the head.  And buddy hang tags are smaller in proportion to its head.  Below, are pictures of Peanut as a teenie (left), beanie (center), and buddy (right):

Also, if there are groups of beanies (i.e. a mixed lot), look at the picture to see how the tags look in proportion to the head.  You can also look to see if some appear to be bigger (buddies) or smaller (teenies) than other beanies in the lot.  In the picture below (on the left), there is a beanie and buddy set.  The buddy is bigger and also has a rainbow star on the tag (not all buddies have this rainbow, however).  On the right, is a picture of a teenie and beanie.  The teenie is much smaller than the beanie, and the tag looks bigger on the teenie.

Do some research on which beanies also have buddies and/or teenies.  An example:  Ronald McDonald has a beanie baby only available at the McDonald's Convention.  There was also a teenie for this bear too.  Be especially careful when it comes to buying beanies that are worth more (i.e. Royal blue peanut, humphrey, chilly, peking, billionaire, etc.) as some sellers try to pass buddies off as beanies, etc.

If you aren't sure if you are actually buying a beanie, ask the seller!  Asking before hand will save you a lot of money and a lot of headache!  If the seller doesn't get back to you, or if the seller is vague or doesn't know, do not bid/buy and go on to the next auction. Remember the phrase, "if it's too good to be true, it normally is."