The cheapest and best format to sell a caravan on is an eBay Classified Ad. This also cuts out the time wasters and non-payers.
A Classified Ad costs £19.99 and there are no final value fees if you sell your item.
Your listing runs for 30 days, during which you can arrange for potential buyers to view your item, you can then end it once paid. You can accept offers too (you don't accept the offer until the buyer has been to view first).
http://pages.ebay.co.uk/help/sell/classified.html
If you decide instead to list in auction format or fixed price Buy It Now it will cost £14.99 for your listing, and that fee is payable even if your item doesn't sell or you end up with a non-payer, and if you sell your item, there are final value fees of 1% of the final transaction (min. £25, max. £45)
