The answer is pretty simple: Give *great* customer service.
Too many creators are paranoid jerk heads. If someone makes a mistake and buys something they cannot use, why not give them a refund? Sure - they may be trying to cheat, but so what? Maybe only ONE out of every hundred people who have a problem may be trying to cheat you. So what? It doesn't cost you ANYTHING to give another copy or a refund.
This is my philosophy and all I ever get are 5 Star reviews - sure, they like my products, but more importantly is when they give me "rave" reviews on my customer service. This is the most important thing because now I have word-of-mouth advertising which is the most valuable and you cannot buy that. They tell their friends.
I *ALWAYS* answer my offline IMs immediately, I *ALWAYS* tell when I will be in world next time and offer to help them and make things right. I *ALWAYS* promise them that if I cannot do anything to make them happy that I will give at least half their money back - and it has never come to that even once in ten years.
Even if a customer seems rude, you - the creator - MUST remember that 1) they are frustrated and 2) Chances are that you are reading what they write as being rude and that they never intended that. YOU should always do whatever you can, quickly as possible, to satisfy your customer. Because when a creator interacts with a customer they WILL tell others about it, positive or negative and whether they speak positively or negatively about you is in YOUR control.
It costs you nothing to make copy after copy after copy of your creation: so just throw a new copy at the customer, no questions asked. If they are cheating you it's not a lost sale - it's a sale that never would have happened anyway, but if it's a genuine customer the good-will in their comments about you is priceless. In the case of selling "classic avatar" items: you need to make it absolutely clear in your description in all caps at the top: "NOT FOR MESH AVATARS". Many very old adverts (created before mesh came along) do not specify. This is why I only shop Market Place and never, ever buy anything I cannot see in world or try as a demo. Ever.
So just give the customer their money back and leave them satisfied. The word of mouth is priceless.
Only one in a thousand customers is genuinely a "bad customer", the rest are just frustrated people who want a solution and a perceived problem resolved. Only you can resolve it or tell them 'screw you" because if you don't resolve the problem to their satisfaction, that is exactly what you are saying to them.