"Do you carry this product?"
End of conversation. End of interaction. End of sale. End of business.
There are millions of products out there. You have 5,000 in your store. The chances are pretty good that your customers will ask you for something you do not have. How your staff answers goes a long way towards your success.
Do they ask why?
"No we don't. What exactly are you looking for in that product? Why do you want that product? What are hoping that product will do for you?"
Do they offer alternatives?
"No we don't but we do have this other product that I actually like better because..."
Do they give explanations?
"No we don't. We used to carry that product but had too many problems and switched to this other brand."
"No we don't. That brand is only mass-produced for large chain stores. Let me show you something of which you probably haven't heard that does the job better."
Do they offer help in finding the item?
"No we don't carry anything like that. Would you like me to call this other store for you to see if they carry anything it or anything similar?"
If you are hiring friendly, helpful, caring people they might already do this just because of who they are. If they are not, then you need to train them that when the customer asks, "Do you have...?" they need to know how to respond.
PS You are probably thinking, "Aren't we all hiring friendly, helpful people?" In theory, yes. But in practice most store owners tell me this is the hardest part of their job. It was for me, too. Until I learned a secret. Quit hiring for experience and start hiring for character traits. You can read all about it in my book Hiring and the Potter's Wheel: Turning Your Staff Into a Work of Art. The book is $19.99 plus S&H. Far less than the cost of one more hiring mistake. You should buy it. You can read it in one sitting. Yes, one sitting. It will make a difference.