If you can make a test purchase of your product using Free Checkout and download the file immediately following checkout and find it stamped, then you should be fine.
With all the problems you've been having with Stamping your file, it sounds like your PDF is simply far too large and/or complex for the stamper to handle effectively, to the point where it doesn't merely take an unusually long time to stamp but occasionally bogs down completely and gives up, resulting in the failure notices you occasionally see. Discussing the matter with Development, they really hadn't anticipated that anyone would try to sell a PDF eBook that huge as a download, so your file was apparently the first trial of the Stamper on that scale.
I presume you've performed a thorough Optimization to get the PDF compressed and flattened as small as possible? Are you embedding the photos as compressed JPEGs (rather than, say, lossless TIFFs)? Computers typically render monitor displays at 96ppi (which is how SecretPoliceMan arrived at that figure), so there's really no need to make the entire PDF any higher-res than that.
If you need to provide higher-res examples of the photos themselves for close examination at high zoom levels, an idea just occurred to me: make the photos in your actual eBook low-res, and provide a link in your PDF (maybe make the image itself clickable as a link) to bring up a higher-res version of each photo in the buyer's browser.