Read all that there is about them and Then you will know what it is YOU want to buy. You will have lots of variations to chose from. With the knowledge that you gather, you will Know what to look for, because you already know that you want to buy a Correct trapdoor, whatever it is.
So to answer your question, you should look for an All Correct trapdoor. Now you can start reading up on the subject. When you know enough, you'll know what it is you are looking for and at.