I was in the same position and I was offered a pitence for my 505 so I sold it on ebay and made £400 to boot and promptly bought my 510 from there too.
You'd make more money buying and selling privately if you have the time, part-exchange is a more expensive option usually. I've always done it with no problems,just check a private purchase over thoroughly and ask for the service history and log book. Otherwise any Ifor Williams agent should help you, google for your local dealer.