I ran an e-commerce shop for around 2 years and the website had been set up before I took it over. For the first 6 months it was running on osCommerce. And it was pretty painful to make even minor changes. There've been no releases for well over 2 years at this point and I would be inclinded to say that as a official project it is dead. It keeps going through community support with an absolutely massive array of contributions.
Of course none of the contributions are tested, or have any guarantees of interoperability so to install more than one requires both a high level of technical knowledge and a solid backup system. (which you should have anyway)
After the 6 months I changed over to Zen Cart. This is a fork of osCommerce but there seems to be some life in the official version and the code/templating system is at least semi coherent. Again there are tons of community contributions but again you need to know what you're doing with them.
If I were to do it again I would move the whole thing over to shopify or an equivalent service and let them deal with the technical side. All you as a business person should be doing is selling your products/services, not dicking around with php files trying to change language settings or introduce new modules. (of course as a geek this is what I enjoyed about the work and why I've gone back to my natural habitat coding software)
I didn't do this at the time because I was concerned about the cost but once you count in hosting, ssl certification and credit card handling charges, shopify (or eqivalent) would have made more sense. The only other issue being data entry but I'm sure there were automated ways around that.