I'm not familiar with the Wordpress Shopping Cart. I tried searching for press level options based on users or groups, but couldn't find anything. Other shopping cart systems I've used have options called "Price Levels." You assign a price level to a user then set up different prices on the item record.
For example, say I have a normal and wholesale price level. On the item record, I'd set $100 for the normal price level and $80 for the wholesale. The webstore would be set to a default price level of "normal." I'd create an account for the wholesalers and set them to the other price level.
Do some Googling for price levels. Magento may be a good option (http://www.magentocommerce.com/boards/viewthread/108/).