Sales are all computerised these days , all the sales are logged
Just exchanging for something else will mess up their inventory , 7/11 would have had scan the items at the till to register them being sold , you just taking items would have been theft and the shop assistant would have been complicit in the theft had she allowed you to take them .
She did the right thing by scanning the returned goods and then scanning the sold goods , otherwise the records will show one bag of bin liners not in stock and whatever you took without paying still being in stock