it is not blathering at all and the questions you raise are those all sellers of immaterial things/electronic wares have to concider. I've been having almost the same idea for some years, but haven't gotten any further than making a layout for the site and buying a domain. In Denmark we have a saying that translates to "many small streams make a big river", and I believe in that concept business wise also
Concidering previews - watermarks can make your map safe from downloading, no not 100% since you can with hard work erase a watermark in photoshop, but the low price should deter you from that.
Reselling shouldn't be allowed - ie, so if you see the map on sale some where someone is breaking the law. (period)
Yes, some people will print maps for their friends, but I'm guessing most wont since they probably play in the same group. (and one should also cater to peoples sense of morale ie. we can't sell at this low price if you don't pay). And there are other ways to ensure loyalty too
But there will always be someone who will upload the map to some pirate site, its just a question of when... and thats probably few minutes after the first map is sold *sigh* which brings us back to morality