The customer is always right. Not true. Some customers deserve to be right, others need a little help in the manners department. Most customers are honest, fare, caring and all around nice people. Then you have the other half. The customer whom no matter what you do for him or her, is not going to be happy. They will expect you to jump through hoops for some of the most insane reasons (their own, of course).
Example: They come into your store looking for a product you sell. They refuse your help when you offer it. They then proceed to open the box of the product they want to buy, pick through the contents inside, put the box back on the shelf open and choose an unopened box to purchase. Then two days later come back to your store to return the item, stating that it's not what I need. Now you're not only stuck with one open-boxed product, you've got another box you can't even sell. I don't know about most folks, but I can tell you I won't buy anything that has been returned. Not even at a discount.
You have to decide for yourself which one of these groups is worth the effort. If you chose both groups then you understand that it's just how business works. Don't take it personal.