[Django]-Python Webshop for Retailers / Distributors

mezzanine looks like a great project to look at it uses Cartridge for it’s Shopping cart module. They have a really nice demo site that you can use as and end user and and admin to edit products and prices. Django packages also have an eCommerce page worth looking at that evaluates a whole bunch of django packages on different features. I’m sure with those two resources you’ll find something worth using as a base at least.

Have you looked at Satchmo?

Satchmo has support for user-group based discounts:

Satchmo supports setting different price and discount tiers based on
user groups. The most common reason a store owner may need this is if
they want to offer different discounts or prices for a class of user.
For instance, a user with a “Gold” membership may automatically get a
percentage discount. Another common usage is for wholesale versus
retail pricing.

and the discounts may be activated when a certain quantity is ordered:

On the product variations, the price can have an expiration date and/or a quantity. If either of these fields are set then that price is only effective for orders placed before that date, or >= that quantity.

Satchmo is also widely used, so the documentation and the community is good.