Is the garage sale mandatory? It seems to me that donating their things might be easier on them than watching people pick through them, possibly sneer at them, bargain you down, and so on. If they're donated, they can maintain the shiny illusion that other children will respect and play with their toys, and they can feel generous.
And if you donate, it's _over with_ in a very short period of time, instead of them watching their toys bleed away one by one. Also, if anything doesn't sell at the garage sale, you'll have to fight with them _again_, because they'll campaign to keep them.
So I'd recommend a quick donation.
On choosing what to donate, one thought, that you may have already tried, is to pack them _all_ into boxes labelled "Charity", present the kids with two or five or however many empty boxes each with their own names on them, and let them pick that volume of items out of the Charity boxes. Maybe having their stuff in boxes would communicate that yes, this _is_ happening? And that way, their actions would be to rescue toys instead of condemn them.
It also might help a little for them to see what things of your own you're getting rid of - kitchenware, clothes, books, whatever. If they understand that _your_ toys are also going away, they'll still be upset, but the resentment might not last as long.
Another thought, that I think I like better: You could, instead, pack both the Charity boxes _and_ their boxes, based on what you think they're most attached to. Then, once you've packed them and know what fits in them, you could temporarily unpack their boxes onto the shelves in your current home and let them play with just those toys while you're getting ready to move.
Then you hide the Charity boxes away somewhere inaccessible to them. If they want a toy back from the Charity boxes, they bring you a toy of a similar size to swap for it. A day or two before the move, you finally actually take the Charity boxes to charity - and you make sure that they know exactly when the boxes are going, and when it's too late for any more swaps.
If they don't swap, they go with your decision, and they may actually need that - they may need the responsibility to be yours, rather than theirs. To you, these are things; to them, they're friends, and they may need someone else to be responsible for throwing their friends out of the lifeboat, so to speak.