If a pokemon requires a stone to evolve it most likely has moves that it can no longer learn after evolving. This means that when you're trading for a fully evolved pokemon you do not know if it can re-learn these moves and the person can easily lie to you.
An example would be trading for a Raichu, if the Pikachu was evolved before level 58 this Raichu can never learn Thunder.
Let me know if you have any improvements or different solutions regarding this issue!