This is obviously not the shippers fault, hurray for the dimwit who takes the amp apart like every other copycat on feeBay, and can't put the screws back in or tighten them...
What I'm saying is whether you fix it or send it back, this idiot needs to pay you out of his pocket, if he doesn't you're screwed, although you might have some recourse through PPal or your CC.
I'm pretty familiar with shipping damage claims, a friend of mine with the worst luck eBaying has gone through a few. UPS or whoever it was will take one look at the box (not damaged) and the actual damage (inside the amp) and will instantly rule that it wasn't their fault, which it wasn't.
Also, another technicality that they will go on is the weight rating of the box, 100 lb amp in a box not weight rated for it will instantly disqualify you from any insurance claim.
Bottom line is, and I've sent many amps, pack it to survive a drop out the back of a truck... Insurance won't bring your gear back, if they actually even pay and I haven't heard of many cases where they do.
What a SHAME, such a gorgeous piece of gear violated like that! Look at the seller's feedback. He's successfully shipped what, some computer memory? I have to say in closing that it is YOU, the buyer that has the responsibility to MAKE SURE that the sender packs it right, if you have any inkling that he's inexperienced at this, either coach him though it, or request to have it professionally packed.
In final (final) closing, I have to say that "professional packing" is crap, and they will do it as cheaply as possible. I had a 77 Marshall JMP head sent to me, packed by Mailbox etc or similar for FedEx, what a joke, two tiny pieces of padding against the ends and packing peanuts in a weak a$$ box with holes in it when it got here, freakin miracle it made it at all, not even a fragile sticker on it and the head showed up here not working...
Many people know this, and I've had a seller refuse to send a vintage Marshall Combo because he knew full well that taking it to a place to get it packed in NY where people are "angry, underpaid, and don't care" would be the end of the amp...
I just send a Mesa Tremoverb out the other day, 120lbs total weight when I got done! That's 20lbs of packing material around it took me two days to be satisfied with the job, double boxed of course, just about 2 rolls of tape, also I use a monster crown stapler and staple the living daylights out of the box on both ends. Do you think any "professional shipping" center does that?
PS Styrofoam might protect an amp from external physical damage, but it will do nothing to absorb shock...