Here are my thoughts based on my searching and buying experience over the past two years. I just purchased my convertible in April of this year after two years of looking.
- late fall/early winter is a good buying time as people who are selling either don't have the space or money to store it over the winter or are just looking to sell a car they bought to enjoy the summer in
- February or March is a good time to buy as the weather is just about to get warm (or April when it is somewhat warmer) because the market becomes saturated with convertibles for sale (this year there were 30+ e46 convertibles I looked into, last year there were over 40) and thus a buyer can score a good deal from a desperate seller (like I did)
- Selling in the summer time can be a good idea as well as I noticed there were not many convertibles for sale then and the ones that went up for sale sold quickly... E46's were selling for anywhere from $1,000 to $2,500 more than comparable ones I looked at in the spring and fall
In the end, as a seller, just post it for whatever you deem reasonable and see what you get - there is a right buyer for every car.
That being said, not to thread-crap but I echo others opinion that your expectations are rather high even with 40K... The links I have on comparable cars don't work anymore (autotrader and craigslist) but from my spreadsheet of cars I looked at over the past year:
2005 330Ci 81K manual $16,995
2004 325Ci 80K manual $15,500
2003 330Ci 75K manual $15,000