Third time that a PC game is not to be found

Went to Incredible Connection at Somerset Mall Thursday 05/03/2015, took the DVD Cover of a PC game off the shelve to pay for it, assistant went looking for the DVD, 15 minutes later returned to say there's no DVD to be found in storage even though their computer shows that they are supposed to have 2 in stock. This is the third time this has happened at this store over a period of 4 years! Previous time I had paid using my Debit card and then had to waste more time waiting for approval for a refund. This time I insisted not to pay until they located the product. It appears that someone on the staff is helping themselves to some of their stock and nobody seems to care. My complaint apart from my time being wasted is why is there no control as to what is on display and what is in their inventory.