I've been with Infopia for several years, and I'm very satisfied. I've used them to sell on Amazon, Overstock, eBay, GBase, Shopping.com, PriceGrabber, and Shopzilla/BizRate. If you sell repeatable inventory, Infopia's Marketplace Manager makes it a snap to update, list and sell your inventory. Their bulk import/export tools are fabulous for updating inventory fields in a flash - I couldn't manage a large product catalog without it.
Their Configurator tool helps me differentiate my listings on eBay, and it helps to generate incremental sales increases. I also use multiple listing templates that I can quickly update when my terms or pricing change.
Two keys to scaling your business are efficiency and automation, and there is plenty of each with MM. If you've never had a standalone storefront, a very nice one is included in the price, and that can help you break away from being an eBay-only business.
In fact, I'm such a big fan that I've added two pages with information about Infopia to my storefront: