Are you sure your total gold is actually increasing so much. I tried this, for example, I put a stack of 5 items (12 cp each) in the sell box and it showed 60 cp trade value. I removed 4 of them and it still showed 60cp in the trade view, and I clicked trade. But when I closed the trade window and checked my gold level in the inventory screen, it had only increased by 12 cp for the 1 item sold.