What tends to happen is people see something they have sell for ‘good money’ and jump on the band-wagon and list theirs. Currently we may have a plethora of spaceframes listed for some reason. But there really is not a ‘scarcity’ of F-frames. I think there are plenty of them still to be uncovered in sheds and garages (as well as mined from Moultoneers ‘collections! 😉 ) .
But be careful as there are a fair few out there that might be a bit ‘far gone’ to be re-cycled – I had acquired an old Series 1 Deluxe that I have recently dismantled and on dismantling a VAST quantity of rust came out of the frame tubes! Each frame really does deserve inspection before paying ‘ebay prices’ for – they have not all been stored in the dry, low humidity and not all were preserved, well greased, before being abandoned……