Well my question is not easy i guess. I hope to find some answers here.

My idea is to build a website that functions as a marketplace for software.
I'm talking about 3Dobjects that 3Dartists use in 3Denvironments to enhance
their scenes. Visitors can become a member and buy those 3Dobjects. After
payment via f.i. Paypal they can download the needed items. Members can also
sell objects they made theirselves. To do that they must be able to upload
objects to the site, along with previews of the objects, set a price they
want to receive. When an object is sold a percentage of the price is for the
owner of the market and a percentage will be payed to the artist that made
the object. My guess is that uploading, downloading and payments need to be
done automatically so business runs smooth and all members are happy.

That is my idea in short. Does anybody here have some advise in how to start
with a project like this? A plan/model in organising this.

To get the idea of what I mean I can show you the example of another famous
site that does this http://www.turbosquid.com/
It is not my idea to make it that large but it gives you an idea of what I
am looking for. I do not want to write articles. I do not want to start a
forum. My ideas are much more modest.

Thanks for any answers in advance