I've bought a second-hand Brompton for my wife over the internet from a private seller, and would be grateful for pointers on how it should be boxed for shipment, so it arrives intact. It looks to me as though the handle-bar riser could get bent out of shape at the headset, if the bike were simply boxed up as-folded. I would send these pointers to the Seller.
Ask the seller to protect the rollerwheel-stays/rollerwheels. The only serious damage I heard of, are bent rollerwheel-stays from people who fly with the bike often and that, since it is a frame part of the rear triangle would be a substantial damage.
The steering column should be fine. I would also ask the seller to fold the bike completely and to screw in again the wingnuts of the hinges almost completely. Not tighten them so that they can move out of the way if pressed but almost in so that they don't stick out either. Another option would be to screw them out completely and put extra
Ask the seller to loosen the shifters and brakelevers a little so that they can turn instead of brake, when pressed at
Ask the seller to box the bike with a little protection.
The older folding pedals can scratch the frame when folded getting in wrong position - so protection there or better detach the pedals
I recently had a brompton shipped in a cardboard box without any further protection like above and the bike rattled in the too big box, the box was also damaged. Bent came the fenderstays, the front fender and a (aftermarket) clamp which hinders the rear wheel to fold under. The rest was fine - brommi is a very sturdy bike, you can probably relax, congrats to your purchase