Of course it's legal!
Things you'll want to do though before it gets shipped.
1) Check what import duties (tax) you'll have to pay when it comes into the destination country. (you customs department will tell you)
2) Well thats abou it to be honest :-)
Get the person to ship it via FedEx or DHL (or some other big company), they've done this sort of thinga million times before and will give you all the forms you could ever wish to fill out .. trick is you need to get the shipper/seller to fill those forms in properly for you, alternativly you can pay for DHL to collect it from the seller, and they'll make sure it's wrapped up and the forms done correctly.
Also make sure the multiple copies of the documentation are on the outside of the box, and that they describe EXACTLY what is inside, state in numerous places on the docs that it's a SECOND HAND Amiga and you might get away with no import duties at all.
Just noticed the shipper is you .. makes it easier :-)
I had an SGI O2 with LCD shipped out from the states to the UK (bought via eBay), had one phone call from customs, paid them their bit of tax and that was it.
If you follow the rules, it's easy .. deviate from the rules and it gets harder.