Earlier quoted context omitted.
It may not be unreasonable if the overhead to rent is high. It's coming down, though I have no idea what ranges are "too high" or where things currently stand relative to that.
It's not even the monetary cost of renting, it's the time cost. If I have a pickup truck the time cost for me to buy something off of CL is driving there, inspecting it and then buying it. Done. If I have to rent a pickup to transport it back then I've got two choices. I can either rent the vehicle prior to looking at the item or after I decide to buy it for sure. If I always-rent then I'll kill at least 2hrs per CL…
This is stereotypical penny wise pound foolish behavior. WRT to OP's relative hauling some plywood occasionally, for the price difference between a commuter car and a monster truck he could find a young man, pay his living expenses while he completes a complete carpentry apprenticeship, and pay the young man for some materials, and the young-ish journeyman could come out and do all the work for him as a thank you.
Another way to put it is the price differential is about 3x what I paid for a new roof on my house... all to haul some plywood...
When I put in a patio I had a flatbed truck deliver two pallets of bricks, two pallets of gravel and a pallet of sand bags. The delivery fee was roughly the delta between one months payment for a car vs a truck. You'd be surprised how much labor you can get out of a truck driver for $200 or $300 or so, and he was probably happy with me because the job only took him maybe 2-3 hours total.