Due to the nature of the motocross sport, there is a very limited selection of used motocross gear. It is highly advisable that you purchase new equipment due to the fact that most used equipment is severely used, i.e. damaged.
References:
http://www.motosport.com/category.jsp?path=-1|4268&id=31985&mnu=p1
http://www.motoworldracing.com/thor-06-ac-glove.html
http://dirt-bikes.ridegear.com/shop/Chest_Protectors.html