Well, you can just release a product with promos and let the sites marketing team do their bit, but nothing stops you from starting a product thread to gain traction or add a promo vid showing the product in action. I have found that extra effort has really helped. People know before hand what is to come and it generates some hype instead of just being drawn to their attention when it is released in the stores. So yes, you do pay these sites to handle marketing for you, but nothing stops you from giving your work more exposure. That does require more effort, but that is strictly up to you if you feel it is worth it or not.

Nod Nod! Most successful vendors do a good amount of that. Preview the product, create some anticipation, give your user base some chance to interact with you, stir some interest, run a contest. All those things help with building a following. It is also possible to build a following without this - some vendors do it, some don't.
Personally, I like to do it, I enjoy a degree of interaction with my customers. I'd actually like to do it more, but have to limit my time at some point or it becomes cost prohibitive. On a larger piece, I'll typically spend a day to two promoting the product (not necessarily all in one lump of time) To do it the way I would really like to do it, it would take 50% of my time, and that is not affordable, because it would not increase my sales enough to cover the time investment. As a vendor you're constantly evaluating if something is/was worth doing or not, and whether it is a short or long term investment you are making. Some things pay off quicker than others.

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Renderosity Store  Personal nick: Conniekat8
Hi, my name is "No, Bad Kitteh, NOO", what's yours?