Distinguishing scammers from real patrons

Lately I've had a number of enquiries from people overseas interested in purchasing some paintings...in the end they turned out not to be real. Luckily no one took any money from me. I quoted my work exclusive of the shipping costs. When they said they had a shipper they used I started investigating if the shipper was real...when I found nothing to corroborate the shipper's identity. I made it clear that all costs for shipping would be paid by the buyer directly. Once I said that, they were no longer interested.