I think we just figured this out recently and have a fix since yesterday:
It turned out that on some Macs (none of ours), GarageSale can no longer access passwords and access tokens when the screen lock is active. We have no clue why that is, but it took as a while to figure this and change the code, so the GarageSale stays functional.
This version should no longer be affected by this issue:
https://downloads.iwascoding.com/downloads/GarageSale_9.1a14.dmg