Using GarageSale Version 7.0.13 (827), macOS Version 10.12.6 (16G1212)
Have a Smart Folder that shows items with Duration <> 3 days. Select item in Smart Group. Change Duration to 3 days. Crash, Application Quits. This DID work in GS6. It should change the duration and remove the item from the Smart Group resulting in Nothing Selected. Crash report just sent.
Any chance you can reproduce that crash and sent it to me.
I searched our crash database for an crash report originating from the e-mail address you are using on this forum, but nothing turned up. On the other hand, the search functionally of the service we are using is leaving a lot to be desired.