The Mysterious WV YouTube channel recently covered this case too, and it's also worth checking out!
My understanding from that coverage and this thread is that Marsha was essentially involved in drug trafficking and money laundering. If the last person(s) known to be with her weren't involved, could it have been a local gang?
Reportedly, Marsha wanted to purchase and renovate a local theater. What if the gang felt threatened by this because now Marsha would have more of a monopoly on drugs and cash flow in this small town, so they decided to eliminate their competition?