this is the dumbest thing ive read in this thread, besides the OP texting the seller. the envelope contained the sellers possibly activated sim card and other info pertinent to his original purchase. they will do the buyer here no good. and even if the sim is deactivated, it cant be of any use ever again. so who cares he took it. i would have too if i was the seller and if i was the buyer, i wouldnt have cared less. you are uninformed.
and if you really wanted to make a car analogy, its lets say he bought a used car and the seller left a personal item in the vehicle which he forgot about. since you paid already, you gonna be a jerk and not hand it over?
Who said it was a SIM ??? Not the OP
Like i said, why would you pay for something and then let the owner take something from you after you paid !! His documentation or account related items should have been removed prior to sale !