Though possible , I don't think this is very likely.Your observation made me think another possible scenario could be someone in the neighborhood unhappy about the garage sale itself.
My guess is that most perpetrators in targeted "issue" type killings go through a build up. For example repeatedly confronting people about garage sales and ordinances, seen taking down signs, repeatedly calling the city about garage sales and real or imaged ordinance violations etc. This things do not seem to have occurred.
Likewise, issue killings might also focus on what the perpetrator feels is an egregious violation of his / her "issue". For example, victim was known to run garage sales constantly and was in effect, running a week end thrift shop from their garage. The victim does not appear to have done this.