what i'd do:
1) take all power tubes out of the amp, wrap in a crapton of bubble wrap, and label each one
2) pack the amp in a metric fuckton of bubblewrap before it goes in the box
3) require the buyer to take out full insurance on the item
4) require the buyer to pay for all the packaging. You will need a lot of bubble wrap
5) you can do the thing where the buyer transfers money in a way where they can't pull paypal bullshit. international bank transfer is a valid option, and with the fees involved might be a good way. also, all they need is an iban number, which is safe to give out. ask your bank
otherwise, i've shiped a rivera TBR-5 which is a stupidly heavy power amp with 8 6550s in it in the bubble wrap method. I basically put about 2 inches in depth of bubble wrap around it to the point where it could literally bounce and absorb all the shock. Then put on an outer layer of gaffa tape to hold it all together. Worked like a charm.
Then box it.
Packing in such an over the top way should also comply with packaging requirements that shipping companies have as well.