When you use other people to sell your stuff, they are going to take a cut of it. And then you may be locked in with their quality and limitations. You can't run a 2-for-1 promo if you can't discount the price, for example (I don't know specifically with those companies, just in general). The more you can keep in house, the more flexibility you'll have and the more money you get to keep for yourself. If you do decide to use a company to create your products, try to stick with one company so you don't have to manage a ton of different locations online. You want to keep creating art, not fiddling with different websites.