You can have all of your applications as long as you have updated it (syncronized it) with your iTunes account. If you haven't updated it, I would recommend you do so just prior to selling. I would then delete everything from it before selling it. When you get your new one simply syncronize with your iTunes account and they will upload to your new unit.
If you are selling the iPod you will first need to reset it to its original settings and erase all of your information off of it. Your itunes library will become the new owners library but that is where you would be able to find all of your apps. What you could do is plug in your new ipod sync it with the old library.