Many traders use Autotrader, Ebay, Caravans for Sale and Caravanfinder. You can usually see a link to their main website with telephone numbers, address etc. google earth will show you its location etc. Its not difficult to identify the genuine ones, but even here I would not pass any money over even as a returnable holding deposit without seeing the caravan. They may go into Administration, or liquidation whilst holding your deposit. It's not as if there's shortage of caravans for sale at the moment, and it won't change in the foreseeable future.