normal bugs occur in the online trading is whether the 
entered code of particular code is correct or not?

whether user is registerd or not?

checks 4 authorization and access control

1)Billing amt was not coming properly
2)added products getting changed 
3)Free gift items were are not appearing properly based on
the bill amt cross
4)Removal items from the cart not reducing the net bill amt 
5)Total no of items were getting changed finally
6)Duplicate items were coming even though not added to the cart

hope this is very helpful for u.bcz currently i am working
for this