I sold a SX-64 about two months ago and the buyer only wanted to pay for ground shipping with no tracking availible. Now eight weeks later he says he has not received item. I told him to wait another couple weeks, or I guess I will have to refund him or have my 100% feedback marked.
I don't know. If he insisted on shipping it that way, refusing your offers of using a more secure method, then it is the BUYER'S risk, not yours. Hopefully you kept the receipt from when you sent it at the post office. Just to prove you paid for postage on such and such a date. It is up to HIM to contact the post office and find out where his item is.
Also, surface (via boat) mail can take three months internationally.
I think you should not have to re-imburse the buyer for a shipping risk he/she insisted on taking, especially considering registered mail was offered and refused.
Heck, he/she might even have been trying to set you up. The buyer might actually have the SX-64 but is just claiming it was lost. No way to tell because of the way they chose to ship the item.