Can I move app purchases from one Apple ID to another? Can I move apps purchased with one Apple ID to another Apple ID? I am looking at purchasing a used iOS device and I want to be able to keep all the apps that are on the device (with permission of the original owner of course). How can I do this?
You can't, as Apple doesn't provide this ability.

From Apple's support document:


  You cannot merge two or more Apple IDs into a single one. You can, however, use one Apple ID for iCloud services and another Apple ID for store purchases (including iTunes in the Cloud and iTunes Match).


You can keep using all the old apps, but they won't update without the original password for the original Apple ID. In practice, you will eventually tire of the prompts or delete the apps in question. In the short term, you can use them all without needing the other account name or password.