There are great reasons for someone overseas to look for a used cat in the States. It's easy to find the type of boat you want, and prices can be a lot lower than in a small country, where the choice is limited. I seriously considered getting a used I20 and shipping it to Israel, and It still would have come out less than purchasing one of the very few good used ones available here. Dropped the matter mosty because we are now looking at buying a new boat...
As for making sure it's not a scam, a nice conversation on the phone should be enough to convince you he's a real cat sailor, seriously looking for your type of boat, which reduces the risk to what it normally is with most other buyers. After that normal caution should be enough