It's no secret I'm a huge fan of Thieves' World. Growing up, and especially in middle school, anything Robert Asprin was my kind of fantasy. Mix a little humor, with equal parts high fantasy, and add a splash of random nuttiness and you find yourself at one of the greatest fantasy book series collaborations of all time.
Another few authors that have contributed a lot to the series, Jody Lynn Nye and Lynn Abbey, kept my interest in other series all through the 80's and early 90's. These are still common to find books, being available in a multitude of different omnibuses, comics, and reprints. A first edition first printing paperback sells for around $15 on ebay. The role playing game Sanctuary fetches around $50, and anything autographed by Asprin or Abbey can drive up the price a little bit, but the books are not in high demand at the moment. I've had a great deal of luck getting first editions on Amazon, ebay, and thrift stores around my area.
Do yourself a favor, buy these books while they are cheap. They have a cult following for a reason, the world is immerse, the characters priceless and the stories you will remember forever.
*Note* I took all the pictures at my desk in my office. The autographed copy I have is the book cover only, given out at conventions during a book promotion tour for the original Thieves' World. Yes I used my broken iPhone, but i realized it still takes great close up photos!