The obvious - be careful with eBay as there are lots of fakes out there, and you have to be pretty magical to know the difference (I don't know much really)
They only sell two sizes now, one which is Agenda the other which is Pocketbook:
Agenda size has a completely different size rings to a Personal Filofax, the spacing is completely different. The page size is also different, Agenda size is A6, which is very nice and proportional but it means anything you use in a Personal size Filofax will not be able to fit.
See picture below:
Do not buy the Agenda size thinking it will hold Personal sized pages.
If you do go for Agenda size I recommend buying the Rapesco Diary Hole Punch (it is so cheap on amazon) then take the back cover off and put it so both sets of hole punches line up with the middle (thank you Steve for that tip)
Pocketbook size holds Pocket sized Filofax inserts, so that is quite easy (I do not own one so I cannot give much more information on them)
They used to sell an A5 size one, which Anita has done a blog post on Philofaxy about, it appears the ring spacing is personal size Filofax spacing, just to be really awkward.
Here is a paper size comparison from Philofaxy which Steve did back in 2011
Any questions let me know and I will try and help you :)