Sean - you can ship successfully, of course you do have to pack properly. I would recommend using polystyrene for the front and back of the amps as sheets of it can easily be cut/impressed to accomodate switches, knobs, binding posts etc (the Yams are problematic as their binding posts are mounted to easily-breakable plastic forms and not onto the metal case, and they stick out a lot). I'd then wrap in plastic-wrap to hold the poly in place, then in five or six layers of small-bubble bubblewrap or two layers of giant bubble bubble wrap. Always have the bubles on the inside of the wrapping. So far, so easy - just buy your bubblewrap chealy by the roll and 'roll' it on - easy! You can then box at this point - single-wall is fine but it should be a snug fit. Keep the cable plug outside this so the pins can't scrape the side of teh amp of course. This then goes into a bigger box. IMO, overfilling with packing peanuts is best, esp the ones that look like cheetos. You should be aiming for at least 3" all around, so your final box will be bulky, but not necessairly much more heavy than the amp. The key is to *overfill* slightly so that the peanuts are lightly compressed and squeeze together. THat way they won't slide around and therefore your amp won't. Secondly, they still have more 'squish' left in them, so sudden shocks AND penetrations can be accomodated. Solid polystyrene is good at protecting against penetration but it doesn't allow any movement, so the shock of impact goes straight to the amp. Anyway, start by putting 4" of peanuts in the bottom of your outer box (ideally double-walled and with handles). WHen you put your inner box on, its weight will squish these down a bit. Then pack them in around the sides and finally pile them on top, so that they appear as a mound above teh line of the top of the box. When you fold the flaps down, they should again compress the peanuts, forcing them into an effectively contiguous foam that neatly surrounds and holds the amp. Tape up thoroughly. Mark which way is up and that the contents are fragile. Take some digital pictures of your packing to show the initial condition (this costs nothing but makes it easy to show damage that occurs in transit).
All in all, more than five minutes work, but well worth it. I buy and sell a lot on eBay, so I've got pretty good at this and I keep the materials etc to hand so I can rapidly and effectively pack the stuff I ship. For someone doing this for the first time it can seem a bit daunting but it's not really, if you just follow procedure.
As an extra, if you are eco-minded, include a paper bag/sack in the box so the packing peanuts can be saved and re-used later on, use goods recycled from waste (not just 'recylable' which means little by itself). I also never tape up bubblewrap as it's really not necessary and just makes it get torn when your item is unwrapped, rendering it less useful and probably therefore causing it to be thrown away. I reuse as much packaging as possible - it's good for the enviroment and it saves money for everyone
Oh, and for the record, my M-80 was damaged in transit when shipped to the guy I bought it off, in the US. He did a sterling job of shipping it here in one piece (well, no further pieces
). I've also had other stuff shipped over without incident, so please don't feel that transatlantic shipping is overly risky. If you prepare and pack properly, it's well worth it. We brits often cry over the much lower pricing and wider availability of nice gear in the US, so I don't want to encourage a 'shipping is a hassle' mentality!
Good luck selling!