suggest you have a look at the property agent module from Ventrian Systems. The module is probably badly named but it is an excellent multi use module. Some of its uses have used it as a store with payment links to paypal. You would set the module up for each of your individual 'stores' and give the store owner rights to enter products etc into it.

The use of the module is only limited by the imagination. Their article module is also excellent. You can pick up all of the Ventrian modules with source for a very low subscription rate. it runs I believe on both dnn3 and dnn4. The supporting forum will show you how to include purchase links to paypal etc.