Some of the sellers wont post individual trees if they are in the cheaper category of tree. They show a generic picture and if they are a honest seller they will say clearly this is not the tree you will get but a similar tree. The older, more developed (read bigger trunk) trees will usually be listed individually and the description will clearly say "This is the tree you will get".
Bigger sellers will have many smaller trees. they just dont have the time and the website space to picture each and every one individually.
Brussels Bonsai and Wigerts are two that I know of that will clearly say if the tree pictured is the tree you will get or if a similar tree in development will be sent.
What exactly is your price point? If its less than $100 chances are you wont be seeing each and every tree at that price.