I would recommened Tyvek brand self-sealing mailers to anyone who ships any type of clothing or other items that may be damaged by water or dirt. I only occasional sell clothing on ebay, but when I do, I always use a Tyvek envelope for shipping.
Tyvek is a special material that is virtually untearable. It is also water-repellent. As long as the item isn't breakable and the envelope is well sealed, there's not much the USPS can do to the package to damage the clothing or other item inside.
The mailers come in a large variety of sizes and while they work wonderfully, they are little pricey. Since I don't use many, I purchase packs of 50 at Office Max once or twice a year. However, I have seen them at the wholesale club for a cheaper overall price-per-envelope, but a larger upfront expense as you are buying in bulk.
The one thing I always try to keep in mind when using the Tyvek mailers is the strength of envelope and glue seal. They can be hard to open to remove your item. I always try to leave a little space that is empty so that if the mailer has to be cut open with a scissors, there is a place to safely do it without cutting into the stuff inside the package.
Overall, Tyvek mailers are wonderful at securing items during the shipping process and I would recommend them to anyone who ships clothing or similar types of items.