I like the idea that the truck is the warehouse. Of course it's an issue to get an item out if it's buried, but that's just a limitation on how tight you can pack the truck.

Putting things on pallets might let you slide them around and manage the space better. Also, I don't know the weather in SF, but I'd never trust the floor of a box truck to stay dry in the Midwest, and pallets keep things off the floor.

A make-or-break will be how much unsold stuff you end up driving around. I wonder if there's some way you can discourage stratospheric speculative pricing, by offering a better commission if the seller lets you set the starting price.