Craigslist - Most people never think about Craigslist when looking for cheap NFL jerseys, but it can be a gold mine of cheap merchandise. Just because it is an online classified does not mean you cannot sell your kid's jersey there. This method requires some legwork, but it is well worth it.
Kinkos - For those of you who prefer to keep things local, Kinkos has a great selection of pre-owned NFL apparel. They accept PayPal and most items are shipped overnight. Make sure to take pictures of your jersey to prove that it is in good condition. When you are ready to ship, make sure to use a trackable and insured shipper.