When I try to activate my FSDT scenery many of them will not accept my registration key. I am sent to an eSeller (?) website where it generates a key that isn't close in format to the key I have for the scenery.
You are using the offline activation, which use a different set of Serials, which in any case should be self-explanatory how it works.
The offline activation comes out if you can't reach the activation server. If your internet connection is working, the issue is surely caused by an antivirus or *firewall*, which is blocking access to the activation server, so you must turn it off, so the normal ONLINE activation (which ask ONLY for you Serial Number, so it's easier) will work.