"I sold my soul to make a record, dipshit. And then you bought one"
I've always figured that meaning of this line is this: In order to make a record, Maynard wrote lyrics about his feelings, thoughts and emotions - his soul. And then you bought a copy. He sold his soul to you, and you bought it.
So Tool, and just about any other band/singer who gets their/his main inspiration from their/his own life and experiences are selling out, because they are selling their art, and therefore themselves, to you.
If you continue on that note, you might stop to think about how stupid the whole selling out idea is, and if you go further, the song gets to be about how we are all in the same boat and how there are much more important and interesting things to worry about than wether band X sold out.