logic for stock management in c# with MSAccess as backend

Question

hi guys 

i am trying to do a simple  shop management system 

i need to buy , sell few items and to show total stock after every transaction .

i have used 2 list views . 1st for showing each transaction and 2nd for showing available stock 

 I am stuck up with the selling scenario . 

how to get total quantity of each item from database ?

suppose i have added  CAR 10 , trucks 20 and then if i have to add CAR's again how to do that ? i want to show it as a separate transaction but want to add that quantity to total stock . I can show that as a separate transaction but unable to update the total quantity :(

can you help me with the logic  or code snippets ?