To me, making "stickers" is a commodity market and not a custom market like vehicle graphics and signs. In this regard, you'll be competing with the lowest price for any given application, someplace I care to never go with my business. In the 4+ years I've been at this, there's only been a handful of jobs I've done for a large qty of decals/stickers. This just isn't the market I care to be in, but if that's your thing, go for it. If you really want to be able to compete, it will likely be with screen printing as has been expressed already. Sure, you can do it with a digital printer, but they really aren't going to be able to compete price-wise with a good screen printer.