>Options >Security Options >General Settings >Click the wheel and select "wipe handheld." That will wipe all user data and leave the apps and OS intact.
To put it back to factory settings you can wipe with JL_CMDER and reload the OS from scratch. This takes a little more time, however.
Thinking of whomever purchases your BB I recommend calling your carrier and cancelling any BIS accounts you have so that the PIN is not associated with an old account when the new user wants to sign up for BIS. He/She will be on this forum looking for how to get registered when the BB is already registered under another account.
Also, if you look in the Buy/Sell/Trade forum there is a good sticky on what to do if you are buying or selling a used BB.