I would like to build a site where users can upload their own merchandise and add a price on their product. When they make a sale, I deduct a fee from their listing price. For sales under $15, the fee is a single flat rate of $2.95. They would keep the rest. For sales of $15 or more, the fee is 20% of the listing price and they keep 80%.
Are there any modules out there that does something like this? I have Google'd and I am having a hard time finding something like this.
You are describing a multi-vendor store. The basics are already in the core, and it is probable that these will be extended, amd made available earlier to VirtueMart Support Members (http://extensions.virtuemart.net/support/virtuemart-supporter-membership-detail) - see http://virtuemart.net/news/latest-news/460-updates-about-virtuemart-3-support-membership
There is a points based 3rd party extension for multi-vendor but it may not fit your requirements (I have not tried it) - http://www.nordmograph.com/extensions/index.php?option=com_virtuemart&view=productdetails&virtuemart_product_id=60&virtuemart_category_id=3&Itemid=121