1) If I touch something that a hindu previously touched and then touch something else with a wet hand or if I touched something wet would it become najis?
2) For example, when you go shopping, there will always be items that have been previously touched or the person at the till could be hindu and if you then touch that item with a wet hand then is it najis?
3) And is it true that after you touched one dry thing that's najis and that touched another dry item, then whatever else touches the second dry item isn't najis?

1) Yes.
2) Only if you are certain someone najis touched it.
3) If you touched something that was najis but dry and your hand is also dry then najasat does not transfer.