Get packing foam from a store--you know, the pink, fluffy stuff? Get some bubble wrap and LOOSELY wrap the model in it. Line one carboard box's bottom with the packing foam, and place the model on that. Now rip pieces of foam and pack LOOSELY around the sides, but enough that they keep the model from jumping around. Seal the first box, put into a larger box and then add packing foam around the box inside (under, too) so that the box itself is LOOSELY cushioned. You want the model to be able to shift slightly in the first box to absorb small shocks, and the first box to move slightly inside of the bigger box to be able to absorb small shocks too. I sold about 60 models in the last two years through eBay and used this method, and only had two or three small mishaps--things like a TC's mounted MG breaking off the cupola or something.
It also pays to be proactive about maybe including an extra antenna if you have them affixed--they tend not to survive. You can even remove the cupola MG and wrap that separately so it doesn't get smashed. Usually the buyer will not mind superfluing it back on.