Channel Advisor is probably the best solution available for selling across multiple marketplaces. They cover the big ones like Amazon, Sears, Walmart, eBay etc. but have several omissions. They are also by far the most expensive solution. Once you learn their system it is relatively easy to use and reliable. But be careful before signing up. They require a one year contract and may even continue to charge you after the contract has expired. Ending your service through them is difficult. I have also found CA to be very inflexible. They have their process and pretty much require the seller to adapt to it.