I am not a Birdy owner myself but wasn't a prerequisite of folding to shift to the lowest gear (biggest cog) before folding? Or was it the highest gear (smallest cog)?
Other than that, as has been mentioned: Early birdies have some weaknesses on the frame and stem. And this bike additionally looks like it was banged around quite a bit. The manufacturer has quite rigorous maintenance intervals (far more rigorous than any other manufacturer I know) for routinely swapping parts like the stem and the forks, even with the newest models of the birdy and probably for a reason. A breaking stem or fork could lead to a really bad crash. Therefor it is probably a good idea to prepare for some (unfortunately not cheap) investment in this baby, especially if you do not know the history of the bike.