Sell-Buy Date

Companies like Glass's Guide and Red Book adjust their values monthly from information within the industry. They have been doing it for years and have an extensive network of sources.  In terms of selling it's best to avoid times when you know a lot of cars are coming on to the market, and that's generally after owners have updated, which can happen for reasons such as the lease running out, new models in the showrooms, availability of government incentives etc.  The best times to sell are when you know people are shopping -- if it's a sports car or soft-top, sell in the
lead-up to summer.