I don't know much about sales, but I do know how my Dad deals with salesmen, and here are some lessons I've learned from him:
-Try to avoid making statements that don't really say anything about the product. If a customer asks "why is this gun $500 and this gun $800", don't just give an analogy using two different cars, but explain what the difference is between them. Explain why you would pay the extra $300 for the more expensive gun, but also see if the cheaper gun is a better fit for the customer for the price. Remember, it's better business for a customer to buy two $500 guns than to buy one $800 gun, realize the cheaper one would have been a better deal, and find a salesman that will give them better deals in the future.
-As others have said, listen to your customers. If you think .45 is better than 9mm, and someone comes in looking for a 9mm, pretend .45s don't exist and sell him a 9mm. If he's asking what the difference is or which one is better, explain it to him as neutral as possible, and provide advantages for both, to let him make an informed decision (if he asks "which do you prefer" then you can press the .45). The opposite is obviously true if you prefer 9mm, I was just using this as an example.
-Last, if you're swamped, be sure to address customers and apologize for the wait. If you're the only clerk and there are 5 people in line, chances are a few of them are going to get bored and leave. Saying "hi, welcome to ____, I'm helping this gentleman/lady right now, but I'll be with you as soon as possible, please feel free to look around", is a lot better than ignoring them until you're done with the customer.