I don't know of any auto-installer for oscommerce. if you wanted to come up with one, this would be the place to get help writing it.
I know that it involves creating a datbase and granting permissions to a particular user and copying some files.
It's a lot of work to configure oscommerce because
a) oscommerce is complicated - it supports multiple currencies, for instance.
b) commerce is an inherently complicated problem.
given point b, you are probably asking for something that has not been done yet. in order to set someone up, you have to find out all kinds of things....a small sampling:
1) their login name
2) their preferred password
3) their email
4) their bank account / paypal account / merchant account type and various other information
5) how that account is supposed to be accessed...and paypal has 3 or 4 different methods depending on how much you pay them and how fancy your server is. Authorize.net has AIM and SIM. This stuff is REALLY complicated.
6) Do they want to charge tax? How much? on what objects? osCommerce supports a way to specify this but if you want to college the default information, that's one more piece of data.
7) etc.
8) etc.
9) etc.