I'm wondering whether this is somehow possible:
1. Customers put their stuff in the cart and order or save cart or order or ..?
2. Customers want to bargain about their products via chat
3. We'd change the price in the backend for the customers cart or order or ..?
4. Customer is happy and can transfer money
I can't change the price in the shopping cart products in the backend.
And I can't change the product prices after an order is placed.
Is there any other option? Maybe via a plugin?
Try adding an observer for event 'sales_quote_add_item':
Let me know if you need some help with codes.