A cycling question

[mamie]

so tell me cause I want to do this right, if I purchase a used tank, with the fish, filter and everything in it, I don't need to worry about the cycling because it's already been up and running correct?? also, I'm not sure if it's rocks that they have but going to a store called "fins" I just love the soft sand they have if I change the rocks to the sand will it do something to the cycling?