While this SOUNDS good, it probably won't hold up in court (IMHO) very well. The problem is that while sealed, you don't know when the seal was generated. It is just as easy to mail an envelope with the flap tucked inside the envelope, and add the contents later. That being said, a reasonable solution might be (no, I haven't done this) to take your listing to a notary public, and have them confirm your signature on EVERY page. In the US, the notary is required to keep records of their process (verification of your signature) for quite some time. These records (I just re-financed a house) include date, time, and type of document. The notary could care less what the document really is, but they put their 'seal' (actually a rubber stamp) on the page, and verify that you are the person who signed on a dotted line. The cost for something like this is in the $15-$30 range.
As was said in the previous post, this is probably overkill, but if you want to prove authorship, this would work.