I was quite annoyed when my 2" ED barlow that cost me $175 ridiculous dollars didn't fit my 10" dob after having it work beautifully on my 6".
I didn't know things could vary to that degree so much - and they were both Saxon dobs to boot!
Sounds to me a lot like fishkeeping (which I do). With fishkeeping you pretty much get guilt tripped into buying everything for the health of your fish. Much of it does more damage than good I eventually found out.