I'd caution you that both of those sites are 'buyer's services' and that you as a seller, rate about zero in importance. You pay, you take quotes, you bid blind against who knows who (if indeed the buyers are not just fishing), and if you dare try to make a cent over material cost, someone else will undercut you.
Other than that, ya, they are great
First you get good, then you get fast. Then grouchiness sets in.
(Note: The opinions expressed in this post are my own and are not necessarily those of CNCzone and its management)