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 purchase due to having to go to somewhere to rent said vehicle. If I rent-after-inspection I'll save those 2hrs when I don't buy at the expense of making two trips every time I do buy. Neither one is an attractive option.
Two hours according to what they pay me is no joke in terms of cash so in either case if this happens say twice a month I've paid for the total difference between owning a subcompact and a pickup truck: note, gas, insurance, etc.
I don't actually own a pickup truck and most of the time the things I pick up fit in my Forester just fine. But it's not unreasonable to make such a decision when you think through how much of a disaster it is to try and rent a vehicle on a semi-regular basis.