There is one major problem with chain letters, no matter how you received them. They're illegal if they request money or other items of value and promise a substantial return. If you've ever read an Internet chain letter, you probably recognize the statute. Many chain letters claim that under Title 18 they are legal and will actually reference it in the body of the letter. Don't believe it.
Chain letters are simply bad investments to begin with.
Not only are they illegal, but it is mathematically impossible for everyone to make money. Sooner or later, there won't be enough people to support the chain. Selling' a product does not ensure legality. Be doubly suspicious if there's a claim that the US Postal Service or US Postal Inspection Service has declared the letter legal. This is said only to mislead you.