If they claimed it to be "Factory Unlock" and you bought it and still it did not operate internationally, you could have taken it up to them. Given they claimed it, fault is at their end, but could only be realized once you have gone back and informed them of such incident.
Once anyone undergoes such an incident/deal, they must ensure to give feedback to opposite party. Be the opposite party a retailer or a normal person selling anything on an online forum.