The solution you are looking for is highly carrier dependant. What carrier are you trying to use? Payment issues aside, are you able to download and install ringtones through the microbrowser? Some carriers restrict this capability by allowing digitally signed content only. This ensures that the carrier gets their cut of the action.
Assuming you can download ringtones at all, there are a few alternatives. When a visitor agrees to buy an item, the MIDlet can notify the server of the user's phone number. The server can then send a text message to the device containing a one-time-use-only link to the content they purchased. The user can exit the MIDlet, read the message, follow the link, download the content, and the server can then deactivate that link.
It doesn't provide much security. The user could transcribe the link to a PC, masquerade as the phone, download the item, and then install it on as many handsets as they want. Of course they could skip the phone altogether and just run your MIDlet on a PC using an emulator in the first place.
William Frantz
http://sprintdevelopers.com