Well if you already have your mind made up about selling the car, I would probably fix it then sell it. Reason why: With all these newer bimmers on the market, it makes it increasingly hard for a seller to push through and get buyers to buy an older model ( the e39) . And the car not being in total optimal shape would hinder it's sellability, furthermore making it extremely difficult to sell . ** Also depends on the demographic greatly **. Look at it at this angle as well- It cost my friend approximately $800 to get a new ABS module installed with labor and parts, and cost me about $250 in labor and parts for a new radiator. In total thats about $1000 ( per say). Then you also would have to consider the condition of the car ( clean exterior, title, interior, sound electrical components ect..) If in great condition, the car could have a value of 5-8 thousand dollars . --- I know this because I wanted to TRADE my e39 99' 528i to a dealer for a 545i and was offered 7 thousand for it ! . it has 84,000 miles. Im just relaying some of the things he took into consideration before making me an offer. We are very close so he revealed them to me.
One the other hand you can do what "Kai" said and sell it to someone willing to do the repairs , but obviously at a lower cost than if it was fixed and sound.
So lets review the pros and cons
Fixing then selling
Pros- More money (profit from sale) , appeals to a different demographic, generally more selling choices.
Cons- Cost money to fix ( possibly more money than you are willing to invest ).
Selling " as-is"
Pros- Less hassle in finding parts and repairing them, costs significantly less ( if anything at all), could be sold for parts to a salvage yard, dealer, or individual clients, could be sold as a project car, or DIY fix it car.
Cons- Might not appeal to a ( totally reliable/ drivable car ) demographic, less selling choices as a result.
The call is totally up to you. But I would find out how much it would cost for everything to be fixed, then research how much other cars just like yours sell for on the market. The answer should be apparent.
Just my two cents.
I spend at least 2 hours a day cleaning my car