I strongly recommend that you avoid the DIY oscommerce route unless you know a lot about PHP/MySQL or can pay someone who does.
The default osCommerce installation needs a hell of a lot of code changes to turn it into something professional. osCommerce has contributions which can be installed, but contributions/add-ons are in most cases code changes, programmer instructions showing what code needs to be changed where, they are not installation programs that end-users install themselves with one click. The same goes for osCommerce templates, since osCommerce templates are basically the default osCommerce installation with only some visuals changed. Even if you do get to the stage where you have a professional osCommerce site, by then it will be a legacy system and time to rip the innards out and migrate to v3 (v3 is currently in alpha), which will be another major programmer task since by then your system will be unique with its myriad of contributions and code changes.
If you want to get started for 'free' and test the water, I would recommend just selling your products on eBay (eBay isn't really free due to the insertion fee and the final value selling fee, but at least there is no upfront web setup cost, and you have the footfall - the existing ebay visitors to sell to).