The fact that various homeless shelters have been suggested as an "address" for the person of interest, I think we can assume that he does not have any money. I mentioned earlier that it took 4-5 days before the person of interest had a lawyer. That could be due to making arrangements through Legal Aid.
It appears that he is broke and was reliant on his parents for his needs (shelter, food, etc.). If there was a business arrangement with Alvin, and Alvin was set to retire comfortably on the beach in Mexico, while Garland is penniless, I could see that as a clear motive in this disappearance. Garland may have decided that if he gets nothing for the work he did, then Alvin should get nothing for the work he did ... not that it's a rational thought, but it could be what Garland was thinking.
I'm also thinking that perhaps Garland confronted Alvin at the end of the garage sale and demanded money from the proceeds. Perhaps Alvin said that the money was already deposited in the bank, but that they could go together to the bank to withdraw some. That would get Alvin and Garland in a truck together. Perhaps Alvin had a max withdrawal limit ... something like $500 or $1000, and that wasn't enough. I wonder what the forensics in Alvin's truck have revealed.