Haibane may only own used things. Where did Reki get her paints from?
Consumable items (like paint and food) don't count for the "used items" rule.
Though as any painter will tell you, having to use old brushes would suck.
Many consumables are possibly declared used in some ceremonial way and are brought via the Touga's caravans. Considering Reki also smokes, and the town doesn't look large enough to supply itself perpetually.
Nobody ever said that everything that a Haibane owns must be previously used. When the show went over the rules, it only noted that they can only wear second-hand clothing.
Why is everyone so emotionally choked up by the Day Of Flight? I understand it could be interpreted as a metaphor for death, but unlike death, where there's some ambiguity, the Day Of Flight seems to point to an actual passage to a better place.
A friend is still gone forever. Their fate is uncertain, and the fact that you'll never be able to say what you feel is the right thing is bound to put a knot in someone's throat.
That's what the Renmei believes, but no one can ever give a straight answer to what the day of flight is. That's on top of the whole nonsense about leaving without notice. The lack of resolution can be frustrating and depressing.
We don't know if they go to some form of Paradise where all Haibane will eventually go and meet again, or if they are simply ready to move on and reincarnated.
Haibane are physically, emotionally and intellectually human, wings and halos notwithstanding. We see a haibane couple, so it's likely they do the things which lead to babies. This raises the logical question "can haibane have kids, and if so, what are the children like?"