What do you mean by "safe"?
It is not really different to selling them domestically, bar any customs issues on the receiving end.
International shipping requires a bit more experience to get it right, in choosing carriers, having a good shipping rate, and choosing options that allow for insurance (either through your own, or through the carrier).
I ship lots of philatelic and numismatic items worldwide and have done for over 15 years - not so much on eBay, but through my full time job. I am also set to do six figures on eBay this year easily. Sure, there are packages lost, but that happens domestically as well. Customs fees or delays occasionally present issues - so it is good to read up on the customs rules for the destination (or consult the buyer) and figure out what is best.
Just as an example of a totally legitimate declaration issue
E.g. a US $10 gold coin can be declared as "numismatic collectable item" value USD 10 (which is its true face value). It can also be declared as "gold bullion" value $1000 (or whatever the current gold price is). It can also be declared as "cash" value $10. Or for that matter "numismatic collector item" value $50000 (if it happens to be an expensive 10 dollar gold coin of rare date/mint/grade).
All of the above (and some more variations!) are legitimate claims for value and declaration from customs perspectives in different jurisdictions. Some countries require a certain method to be used - some allow you to choose between cash/bullion/collectible value. Also some shipping methods will not allow you to ship anything declared as e.g. "cash", or it may be excluded from their insurance policy. There may also be countries where the receiving country has import restrictions that you as a seller (or for that matter the buyer) are unaware of.
My end point is that, no, it is not unsafe to sell internationally - but it can come with some unexpected shipping issues.
