With the understanding that any tree can be trained into a Bonsai! Pre Bonsai, refers to small trees that are inherently well suited for Bonsai training. Pre-Bonsai trees are the first preference for people who want to nurture a Bonsai from an early stage of development, rather than an already "finished" tree. Small leaf size, bark texture and the ability to create new shoots when pruned, are desirable features. If you are new to bonsai, picking a pre-bonsai tree is a good start.
Pre-bonsai are generally offered at a lower price since it is not yet in its complete shape and form. This is an advantage if you like to personally train the tree to whatever design you have in mind. Often they are still in a nursery pot or sometimes in what is called a training pot. This shallow training pot is to help facilitate the first stages of root training. Great Bonsai starts from what is referred to as Nabari. This the area where the root structure intersects and transitions into the trunk base. It is the most challenging part of a Bonsai to develop and correct if unsatisfactory. Therefore, when you are interested in buying a pre-bonsai, make sure that it is not just an expensive nursery stock, but that it starts from a good foundation. The Nabari should show some surface root exposure that displays the roots are partly trimmed and trained into the horizontal attitude. The first steps in creating the illusion of a mature and established tree.