I've seen various 1800's stampless covers/letters on Ebay. Are there ones far more valuable than others or are they all generally the same value? Excluding, of course, a stampless cover to/from some famous personage.
Yes, there are some with higher values.......ship covers, war covers, covers with intransit markings, interesting destinations, covers from tiny islands are usually highly sought, covers with rate markings, things with great "eye-appeal", many collectors look for early "home-town" covers, and a variety of other things. The main thing would be just about anything that appeals to you for any reason.