Hello
and welcome!! The timepiece you are contemplating seems all legit to me!!! I have had a few of them myself in the automatic caliber 552 and 562 for this type of case. You have what is known as a monocoque case or a front loader which is a one piece case where the movement and dial come out from the front. The DeVille Seamasters were cased in North America like the LeCoultre representing JL and these cases were made there under license by their parent Swiss companies or made for them. L&K was one American company that specialized in case making under license from Switzerland to avoid import taxes on complete imported watches.
These one piece cases were quite popular back in the 1960's as it was an attempt to make the watch more water resistant which was not such a success. That is because most of the problem was through the crown and these have what we know as a split stem system where the stem is made of two clipped in pieces which holds it together. The reason why your seller was unable to open the watch is that it is something which not every layman is able to expose the movement without having the crystal removed and removing the dial with movement from the case for a complete movement inspection ( of course after removing the stem). It took me quite a few years of amateur watch skills to reach this level. One good thing about them if any good came from these cases. As for faking these, I am not sure who would be so brave to go to that effort only to make a few dollars if any?? so your gamble would be quite safe. The generic crystal can present problems if the tension ring is not the correct geometry for this case as dial should sit correctly with the bevel section of the tension ring! As for the price it is ok, but you could do better in getting an automatic version for the same price or for a few dollars more, depending on your tastes!! Good luck with your quest!!!!.