I think I would agree with Trev's relaxed view of what a collectable is - it's whatever somebody finds interesting enough to accumulate, even if others would consider it rubbish!
I know of collectors who are driven, serious, competitive, and not very complimentary about the airguns that excite less discerning collectors. I've discovered over the years that I'm not really like this, although I admire their tenacity and successes.
I have very occasionally bought an airgun without intending to hang on to it, mainly because I'm curious to see it 'in the flesh'. An example of this was an ERMA underlever-cocking rifle. I was impressed, but since it's not my cup of tea, I moved it on quickly.