If you're planning on selling on eBay then you need to know that eBay can and will hold your funds from a sale sooner or later. Your funds will be held even though your shipping costs have been paid out of your own pocket. And even if you are in the right and even though your customer has not contacted you regarding the problem or issue first.
Most customers don't realize that filing a claim with eBay freezes funds in the sellers eBay account. A customer who has difficulty using a product will file a claim stating the "product is not as described." EBay then freezes the money and sends an email to the seller stating that the matter should be resolved. This leaves the seller out the cost of shipping and without the product in the interim.
The more detailed and complicated your product, the more likely you will receive this type of claim against your eBay store. A customer receives the product, can't get it to work properly, and files the claim without contacting the store for help first because they feel more protected through eBay.
The faster the issue is resolved the better---eBay doesn't release your money until the issue is resolved and eBay reserves the right to arbitrate. You'll then receive a letter stating the good news that your seller rating was not affected and your funds are released.