Rare or Vintage t-shirts.  This catagory of items is fun for the collector, buyer and seller.  Some rare older tags fetch a lot.  To identify an older 70's t shirt the best bet is to check the label.  If it is 50% Cotton 50% nylon it stands a better chance of being older.  The most polpular older vintage shirts are name brands like Nike and Adidas or any well known sneaker company.  Some of these t shirts from the 70's will fetch above $20 a pop.  To find them you can hit the usual garage and yard sales.  You can also get lucky at the thrift stores but they ae getting more picked over recently.  Another category that can bring in a big haul is any rock concert or national recording atrist  t shirt.  I recently saw a white concert mid 80's Motley Crew t shirt go for $46 bucks!  I was disapointed as I had just thought the $9.99 for a very similar shirt at the thrif store was too much and I didn't get it.  Finding t shirts that have limited runs and significant nostalgia value can be a fun hobby.  Imagine having an authentic Woodstock t shirt!  These shirts are in limited supply so make sure you verrify the history and label.  Reprints of old style shirts are all over....find the real deal and sock it away like a treasure certain to go up in value.  The better condition the more the value.  Good Luck and most importantly - HAVE FUN :) !