If a salesperson told me I could get a refund for a product and then another denies it when I came for a refund, what can I do?

The most important thing is the receipt.  Does the receipt inform you all sales are final.  If so, you may be out of luck.  The second thing to review are posted signs in the store.  Are there any posted signs stated all sales are final.  Again, if they are, you may be out of luck in your argument regarding a verbal agreement.  

Also, you omitted important facts: When you returned the item after the purchase? Did you have a receipt when you returned? What was the condition of the item you purchased when you returned it? Why did you return it?

Finally, if the receipt does not prohibit returns and there aren't signs that prohibit returns, next time you go into the store demand to speak to the store manager.  In fact, you shouldn't have left the store without speaking to the manager the first time you attempt to return the item.

Also licensed in New York.

The way to resolve the issue is to confront the salesperson who made the representation as to the refund of the product that you want to return and then see what the outcome is. If this salesperson denies what was said, then you should no longer purchase from that store in the future.