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original imac box, should be fine, there's plenty of foam and support in there.
Just make sure whoever you send it with, you pay the extra for the insurance, so you're not out of pocket if it breaks in transit.
I've used Parcel2Go for most bigger boxes and it adds like an extra 10-15 quid onto costs for full cost insurance. Might be more for you. Just set your postage costs on ebay accordingly.
I'd be tempted to get a genuine iMac box even though it'd cost you a fair bit.
I had a cinema screen ruined by UPS before. I packed it up in proper thick cardboard boxes but the internal padding wasn't enough so the screen moved within the box itself. As there were no sign of external force I got no payout.