I am starting a ecomm business and my site idea is pretty similar in functionality to ETSY. I am sure that I could build it in wordpress (or drupal or magento) and just use plugins to get all the functions i want - but is there risk in being dependent on an outside technology rather than just building something out myself (I do not know web development, yet, but am learning) from scratch like in ruby or something?
It depends on your goal.
If your goal is to build a working website that will start earning as soon as possible, use as much prebuilt software as possible, in particular pieces that are known to work well together.
In reality, you will end up somewhere in the middle - some software from scratch, some using existing pieces, all held together by some more of your own software. How much from scratch and how much existing will depend on how comfortable you are with either side.
Ideally, if what you are producing is itself the product - for example, a web service like Etsy as you mention - I'd recommend doing it all yourself and not using an existing CMS. The reason is once you start moving forward, more and more of your time will be spent fighting with how the CMS works to get it to fit to your particular model/requirements. And if the system gets popular, you're going to run into all sorts of limitations as the CMS was not designed for your specific purpose.
That said, since you arent currently a very experienced developer and/or are working with minimal resources, for your specific case it might be best to get something up and online using an existing e-commerce system just so you can get some experience running such an application. And then, if it looks like its going somewhere, after you have the background and understanding of the system's requirements to redevelop the app, you can do so.
Of course, when you pick up a 3rd party product, there's always a very small risk that one day down the line that product might have support dropped for it, and that may or may not impact the systems you use for your business; however that's something which every business faces these days, so it's no big deal. (Well, of course it's a big deal if it happens, but it's a risk which you can hopefully manage as part of your overall business strategy by keeping a fallback strategy)
On the other hand I'd say there's far more risk associated with starting a new project by building everything yourself from scratch. The risks are that you could end up with a product which is unstable, insecure, buggy, etc; and you'll have no support to turn to when something goes wrong - and if your business suffers as a result, you will have noone and nowhere to turn to.
If you're starting a business without having much tech knowledge yourself, then I would think the smartest thing to do would be to spend time researching the kinds of products and services out there which are within your budget, and already widely used by many other businesses in a similar situation to you - chances are you'll have plenty on your plate already without having to worry about building the web functionality from scratch too.
If you want to have a business that runs quite quickly, especially with you not being a very experienced programmer, Id go for the pre built stuff. In addition, it is going to take you a very long time to create something on par in functionality with e.g. wordpress. You might want to spend that time not reinventing the wheel, but really focusing on some great content to draw customers.