I'd have thought now in the run up to xmas would be a good time to buy, people spending on xmas and not wanting any other big expense... added to the fact its closing on year end and many garages will obviously be more willing to do a deal to get it through the books...
as for selling, I dont honestly think theres ever a good time to sell a second hand car over 10k privately... unless its something cheaper than the rest and popular.... I have always tried to sell my cars privately before trading them in, my last car a Mk4 R32 was a nightmare to shift, even though I advertised it competatively priced and it had 12 months extended VW warranty, in the end it went via ebay


. which actually worked out pretty well.