although experience is not my own, it appears that CMS built in ecommerce capabilities might work fine for trivial stores. so far i know, these build in capabilities are not extendable in functionality.

so, if you need a simple store - CMS likely to help. as say paypal can. if you would need shipping, taxes, special pricing or something fancy - you better go with a specialized software. in most cases there's an option to integrate sessions & customer profiles (if applicable) of the CMS driven part and a shopping cart part if the code is open. takes some development though.