For context, I am a volunteer in a charity shop myself, and if I see something that I want, I will purchase it. The shop doesn't give staff discounts, either; staff pay full price - the last item I bought, a book, cost me £40.
Anyway, I COMPLETELY understand people objecting if volunteers were to just take donated items for themselves, but I've seen some online discussions in which people complain about the staff getting first pick of donations when they are paying for them. I really don't see the problem in this case, and think people who object are rather out of line. Other retail workers surely sometime purchase items from the shops they work in, so I don't see why volunteers should be begrudged that. A volunteer is a customer as much as any other.