Looks like you are free to develop and sell using express, see
this thread by Adrian Collier (Program Manager - Visual Studio Express).
He states:
'Provided that you comply with all the License Terms for a particular Express release, the 2010 Express SKUS can be used to create commercial software. Before you register
the express SKU you are effectively running a trial version of the Express product, which expires after 30 days. There is no limit on creating commercial software during the trial period. Once you register the product, the ‘Evaluation’ text
changes to show it is a licensed copy.'