Look at your post with a very critical eye. Ask yourself if it appeals to you, asks you to spend time reading it. There are some very sophisticated mailing programs on the market now, which can provide spell checking, variable line length and margins, and different type styles. The type styles will govern what appears on your screen, but line and page lengths will make a difference in your outgoing mail. It lures you into a sense of familiarity, of casualness.
Email lies somewhere between casual informal speech and formal writing.
The immediacy of sending an email post may trick us into being less careful about how we present our material. Studies have shown that people pay more attention to email which looks good and easy to read. The fact that you are reading this on Biblio means that you have at least some passing familiarity with mailing lists and how they work. Biblio may be the best mailing list for selling scarce books but it certainly is not the only list. There are also scholarly mailing lists which you may wish to look into.