So, perhaps we have a person from out of town, which supports the 2am drive by and the "here and gone" nature of the vehicle. Now we combine this other important facts:
1) The killer knew about the yard sale-not only that, they knew about what type of items would be there.
2) The killer knew enough ABOUT the sale that they could COMFORTABLY wear a disguise/cosplay getup to both disguise theirself AND put Liz at ease as they approached. This is important-if they wore a disguise and this was just a normal yard sale, Liz would have been alarmed to see a costumed person approaching her. She most-likely would have ran or screamed.
We also know that the POI specifically asked Liz about the stormtrooper helmet, lending more credence to the fact that this was an item the killer may have expected to see. For whatever reason, the killer asked how much THAT item was. He could have asked about any of the items-why that one? Was it simply the most prominent item (bright white in dim lighting) as the killer approached?
The killer had to be able to approach Liz at close range without alarming her. The yard sale provided the perfect opportunity for this, yet how did the killer know? Who had this information? It's almost like you had to be there sitting with Liz and Sergio as they planned the details of the sale. There is just too much going on for someone to be THAT lucky.
This was part of the audio analysis from Arrin Stoner’s video.