well ... i've set up and modified oscommerce once, and once is enough for me. it's powerful and the intentions are good, but it is has serious flaws that destroy all the benefits.
1 - there is too much code making the sote difficult to modify - there are multiple database queries which are virtually identical, or they would be if they used SELECT * instead of specifying every single field. adding extra fields is difficult
2 - the design of the code is such that the page headers are displayed before the database is queried for content - this means that you cannot have unique titles and meta tags etc on each page - please don't say it can't be done anyway, because it can. all you need to do is do all the database queries before any output.
3 - search engine safe URLs are a waste of time because of the way sessions and cookies have been implemented - search engine spiders dont allow you to set cookies - turn cookies off and you get PHPSESSID in the URL - search engines don't like sessions in URLs. don't say it's impossible to have a cart without sessions in the URL, because it isn't.
4 - by default, the cart requires people to register for an account prior to making any purchase. compulsory registration is very likely to drive away all but the most determined customers - why would they go through the hassle of registering (and it's a lengthy registration) when they can easily go elsewhere and buy without any hassles?
the above are some of the very serious flaws - there are many other smaller flaws, such as the registration form requiring date of birth etc - under UK (and possibly EU) law, that sort of thing is termed "sensitive personal information" and requires site owners to register under the data protection act. i could go on and on and on.
it owuld be far better to use one of the many other free shopping carts out there that can be configured to suit. even if it means you don't have search engine safe URLs, you can't be any worse off than with oscommerce.