Plain Letters are created when a player makes a copy of a letter they received in the mail. Originally, a Plain Letter could be sold to a vendor for 1 copper but now has no sell price; I don't know which patch changed this attribute.
To create a copy of any letter - excluding Gamemaster/Blizzard mail - simply click "Keep a copy." The icon exactly matches the icon you see on this item page.
I don't think there is much of an Auction House market as there used to be for Plain Letters. I know that during WotLK I could browse a couple pages worth of Plain Letter. Some people would buy them out of curiosity, i.e.: "What could be written inside...?", because there is no way to copy the From and Subject fields of the original letter. (This is particularly frustrating for players such as myself that retain and collect sentimental or otherwise important letters. If I can't remember which row and column... ugh.)