That is a really great question, and I suggest, as well as do many others, that you do not offer international returns. If you are accurately describing and taking pictures of the items you are selling, you will rarely run into this.
Treat this type of transaction the same way you would a domestic sale. If you do for some reason have an international buyer looking for a return, it is honestly best to eat the cost and not have the customer return the product. You will quickly see how expensive international shipping is with a return.
Do not let this scare you away as the increase in sales dramatically outweighs the few returns you may have overall.