Need more information. A 2003 could actually be anywhere from 6 to 26 months old. At this point in the cars life, the remaining warranty period can add significantly to the value as there could be from 22 to 42 months left of the 4yr/50K mile warranty. Since you're the original owner, it obviously is not a CPO car with an extended warranty.
I purchased my 2001 in Oct 2003. Since my car was purchased new in Nov 2000, it was 35 months old and had 35K miles on it when I got it. Mine is a CPO car, so my warranty goes to 6yr/100K miles. I feel my car was in above average condition compared to the other 6-7 cars I looked at before buying this one. I feel I got a good deal at $49.9K, which represents $25K of depreciation over the first 3 years of the car's life.
Even with the low mileage and the actual age since purchased, by the calender year, it's 2 years old, as 2005's are already on the road. If you really want to sell it as opposed to just talking to people, I wouldn't ask over $64k and would be looking to settle for $60-61K.
Aside from going into winter, now is a good time to sell because a person looking for an M5 has to settle for a 2000-2003 model. This means your car is more desireable. By next spring, a prosepective buyer might be more interested in the new 2005 model than the 2000-2003 body style. Also, the basic fundamentals of supply and demand will come into play. Right now supply is low because there are currently no new M5's on dealer lots so if you want one, you have to buy used. Next year there will be a fresh supply of new M5's as well as a surge of used ones coming to market as owners of 2-5 year old cars purchase the new model and sell there old ones.
Just my humble opinion. Hoping to hear feedback from others.