Posted this theory on Reddit but I'd like to post it here too:
To me, it's clear Liz was targeted, it was NOT random. For what, and by whom, I couldn't tell you.
Someone somehow knew that Sergio would be leaving around a certain time that morning and knew that they could strike very soon after he did. How they knew that I think is one of the big clues to this case. To me, this is likely someone close enough to know about the garage sale and what time it would be starting.
At the same time. Another scenario, what if the killer didn't know about the garage sale. What if they would have gone there planning to knock on the front door and would have shot Liz as soon as she answered, and instead found her outside setting up for the sale. I think this is less likely because Liz was safety conscious, and probably wouldn't have opened the door for someone suspicious, not to mention, they would have had to walk up to the porch right in front of the camera (assuming that they knew about the camera), so this again brings me back to the garage sale. Two possibilities here- 1) someone knew about the pending garage sale thru Liz or Sergio directly or 2) They happened to see the sign from the night before and decided to take advantage of it, which still wouldn't explain how they knew Sergio would be leaving at the time he did, so to me, this is someone that knew of the garage sale directly thru Liz and Sergio. They didn't post about the garage sale on social media, so that can be ruled out. If you gather a list of everyone who had advance knowledge of the sale, the killer is either one of those people, or was given the details by one of those people (whether or not they knew or didn't know what they planned to do to Liz. It could have been someone who casually mentioned it to someone else and inadvertently set the wheels in motion)
So, Compile a list of people that had knowledge of both the garage sale happening & Sergio's schedule, and the killer is going to be associated with that list.
I forget, what time did Liz typically leave for work? If she was normally gone by 7am, then that implies even more knowledge, that the person didn't randomly choose that day when Liz happened to be off and by some lucky break catch Liz outside. Furthermore, Liz got the day off in advance, so co-workers could be suspect if they knew about the garage sale and Liz saying "hey I won't be in Friday because I am having a garage sale" but then again, a co-worker being the killer would also have to know Sergio's schedule.