The time to sell that E90 M3 is now, just before the new F80 M3 shows up and take a $5-$7000 chunk out of your selling price.
KBB puts the best value of your car at around $54,000. I would say, sale price is $55,000 and expect no less than $53,000 for it. If you are not going to be in the USA during the sale then you must set some legal documentation with somebody trustworthy that can do the job for you. Or simply take a quick trip and sell the car to CarMax -you may lose some value but perhaps you may be surprised. Only you can determine how much money are you are wiling to lose in here -if any at all. For sure, you should be able to get your $25,000 back if this is done right.