I used to sell articles via Constant Content. But I only wanted to sell under their usage license, because reprints are a writer's lifeblood; and I found out the hard way that CC do not consider themselves obliged to actually enforce the terms of the usage license (e.g. that their customers publish the content only once per payment, that their customers do not change the content nor remove the author's byline, etc.). After I'd emailed them several times about my work turning up on people's websites without my byline (or even with another person's byline, the plagiarists!), they started getting snitty with me: "If it means so much to you, *you* go talk to the web site owner."
Except, of course, you can get banned from CC for initiating direct content with the customers; that would be cutting out the middleman. That's explicitly stated in the forum and in the rules for writers.
I suppose I could side-step the issue of whether CC adequately uphold their own usage license by not using that license anymore--by only using the full rights license and thus forfeiting any expectation of reprints or bylines. But I don't want to not have a byline on all that work. And just on principle, I dislike greatly how writers are expected to adhere to their contracts but customers are not held to the terms of the usage license. And if they are going to set themselves up as a middleman and take a cut of my sales, I expect them to enforce the license they are supposedly brokering the sale under.
So I burned that bridge, and will be eventually reprinting the articles I still have rights to wherever I find an opportunity. Maybe via my eHow account if their contract allows. We'll see.
(Full disclosure: I now do content writing for Demand Studios, who take full rights and pay flat fees on acceptance, payable twice a week via PayPal. I can't reprint, but on the other hand I don't have to constantly police my own "middleman". And there's no waiting around for someone to buy. If I do two articles in a day, I make $30 in a day, guaranteed--no rejections yet, knock on wood. So I find this a more acceptable situation.)