If you kill somebody and there is an (for example) an enchanted item on his body

but you go away an after a day of 3 the body is gone so you can't have the

enchanted item. (For example Lord's Mail)

Is there a place (maybe) where you can pick up these items ????

no, they dissapear from the world.

the best thing to do when you find an item you like is to pick it up straight away, if you you are too encumbered, then you would have to sacrifice other items for it, otherwise it is gone.

one exception is if the item you need or like is taken by a guard when you are caught for commiting a crime, the item can be re-obtained by going to the nearest "evidence" chest, which can be found in imperial forts and Census and Excise offices.

Take the item off the body and drop it on the ground. It will then stay there forever.

You can add any item via the console if you have the identification code. UESP has a list I believe.