Looking at the grainy video footage, the killer's body language makes the killer appear angry. Anger and jealousy aren't mutually exclusive.
The jealousy could have been over S, but it also could have been over Liz's job, her position within the Star Wars cosplay community, or even a friendship.
S has said that Liz's co-workers were among the only people who knew about the garage sale, so jealousy or anger over a work situation would not surprise me. I would want to know whether anyone had been fired, reprimanded, or passed over for a promotion because of anything to do with Liz.
Here's my profile of the killer:
An overweight, physically unattractive female around Liz's age who is loud, explosive, and vindictive.
Someone whom others avoid challenging because they know that they'll be "in for it."
Someone who has friends only because certain people would rather be allied to her than be on her list of enemies.
Someone who thinks that she's better than other people even though no one else would agree and who comes across as a know-it-all.
Someone who could be described as a "Karen" in modern parlance.
If you can identify someone like that who might have a grudge against Liz—because of jealousy or because of ANY perceived slight, no matter how minor—then you will have the killer, IMO.