@lonewanderer you won me over with your compelling photographic evidence of a "run out" and logic.
You reminded me of my own habitual "run outs" taking kids to early morning practices: pajama shorts, a wrinkled and loosley buttoned collared shirt. Barefoot, but with shoes, but- no socks in the car.
I am thinking that a run out perpetrator would come from two general sources:
A. Local, with in the neighborhood
B. Non local, outside the neighborhood.
I am thinking that non local is more likely. A neighborhood perpetrator would probably feature: A build up of some sort ("Barb confronts / "advises" victim earlier), Barb has a history of calling in complaints to police, is known either directly, or indirectly as being aggressive, goes on "patrols" etc. Barb is recognized, or, aspects of the behavior are recognized.
I think your scenario of a "run out" works perfectly:
Its "o'dark:30". "Barb", as usual, is nose deep in reviewing intricate social media exchanges. The dots have been connected and its irrefutable that Liz is the source of the slander, accusation, misinformation, rejection etc.
"Barb" has already played with the idea of revenge before. She has a pretty good idea where Liz lives via a previous search on "cool" people finding sites. Now, its time to act.
Barb dresses for a "run out", tells her current significant other that she needs certain uhmmm "purchases"- now. She grabs his truck keys. Barb is not even sure what she is going to do. Knock and confront her with the "evidence"? The garage sale is a coincidence and a big "opportunity". Barb then murders the victim and slightly baubles her escape.