They might want to try talking to the local LEGO Users Group/AFOL group and see if the group participates in buying bulk from LEGO and would be willing to order for the charity. The members of the group in my area, NCLUG, jointly and annually order in bulk at a discounted rate. It is a slow process, but saves them a bunch of money.
A small charity wants to buy up a lot of Lego (and Duplo) to redistribute to children of poor families. There are some suitable sets, but they're expensive, and you need to buy eye-wateringly huge amounts to get a price break on it.
And the second-hand market keeps value, meaning they can't even buy loads from auction / parent websites or car boot / yard sales for redistribution.
Still, it's a good idea and they're working the problems out before going any further.