At the abattoir or mart, the Trading Standards inspector will insist that animals walk out of whatever transport they arrived in. There are rules about the slope of the ramp, too.
We all take anything that'll fit in the back of the pickup to the vet that way
All my bought-in piglets have arrived here in the back of the (well-strawed) pickup.
Pet lambs moved ditto, or in the dog cage in my hatchback.
But journeys of any distance, to/from the mart, to the abattoir - it needs to be properly ventilated, the right kind of flooring, ramp or somewhere the right height they can walk in and out.