I'll throw in some opinions as one who buys/sells a used Cali once (twice?) every year - IME the only two places you need to advertise are a classified ad on eBay and on here. Both work well and done with some thought and care (ie a very well written fully descriptive ad with excellent photos) and priced right will have a sale normally within days and a queue of fall back buyers should the first fall through. Both avenues have been successful for me each year, however, eBay is the biggest window by far for selling vehicles now. Forget AT etc.
Again, IME the market for Cali's is so strong that season is not important, in fact I would say September has always worked out very well to sell and conversely June/July to buy which is the exact opposite of what logic might suggest. It simply does not matter - people are desperate for Calis' and want them at any time of year as their personal circumstances dictate.