Hi
I just paid £3k for a 510 2004 model, which had hardly been used, from an Ifor dealer. The rough guide seems to be that the 510's will be about £500 more than a 505 and the year dictates the price, so a 505 2005 would fetch about £2500 and a 510 2005 approx £3100. Thats a basic model. I suppose if you have extra's you can charge a bit more.
A good one is worth its weight in gold. Am sure you'll sell it - good luck
Mine is an 03 HB505 that I bought from main dealer in 05 for £2,700, he said he'd buy it back (and signed a letter to say so) for the same price if I sold it within 5 years. So, I'd say about £2,500 - £3,000.
If you're putting on ONO, I'd consider adding about 5% or 10% to the price - in other words, if £3k is what you want then put it up for £3,150 or £3,300 (whichever you think is more realistic) and buyers can then 'knock you down' - this way they feel like they made a good deal whilst you still got the price you wanted. If it's £3k ONO now you'll be getting people haggling for £2,700 and less. And who knows, if you're very lucky someone may come and cough up £3,300 without batting an eyelid!