i would like to start selling cakes but i am wondering what size i should sell. is 6 ", 9", and 12" done more or should i go with more space on either sides and go with 6", 10", and 14"???? what do people usually sell? any ideas...

I do both or a variety of other sizes/shapes depending on the number of servings request (or they're going to pay for) and the design of the cake.

As the previous poster said, it depends on what the customer's needs are......how many servings they need. There will be times when you will want the odd sizes and times when you will need the other. It's not what YOU want it's what the customer needs.
If they want a 3 tiered cake to serve 30 you would not be able to do any of what you suggested. In fact I doubt one could come up with sizes small enough for such a few servings in three tiers.
6, 9, 12 makes a very nice presentation in my opinion. I have seen nice ones in the other combination also. It depends on the design along with the needs/# of servings to show off the sizes.