I drove a 1989 Opel Vectra with leaking valve cover gasket for a long amount of time (until it was 22 years old). Not a problem at all. Even an amateur with some mechanical skills could replace the valve cover gasket, but I didn't bother to do it at all. The issue of small oil leak is so minor. I don't know the magnitude of your particular leak, but if the leak is so minor that you never had to add oil during the oil change interval, I would ignore it. I certainly didn't have to add any oil due to my oil leak. Of course, some environmental folks could say that all oil leaks should be fixed. Also, if you didn't notice the leak before taking the car to get it serviced, chances are the leak is very minor.
However, $1985 sounds like a lot for a simple gasket replacement. For that amount of money, you could perhaps even remove and reinstall the engine at a cheap independent garage! I'm not familiar with the vehicle, but at least valve cover gasket replacement shouldn't take more than few hours at most. Timing cover gasket could be a bit more due to needing to remove some belts. The parts should be cheap as well. Definitely get a second opinion for this! And ask for breakdown of the costs (parts, hours of labor and rate per hour).
As for selling the vehicle: the dealers do check all vehicles for faults very carefully, so you cannot expect to get the same price you could get for a car without faults. If you are aware of the leak, and fail to tell it to the new buyer, you may be doing a crime depending on your jurisdiction. I certainly won't encourage people to take part in criminal activity, so if the leak costs $1985 to fix, you should sell the car for $1985 less than fault-free cars are sold.
In general, if this is your largest issue with the car, I wouldn't sell a 9-year old BMW. They are nice cars, but at least where I live they depreciate somewhat faster than Volkswagens and Toyotas (although not as fast as French and Italian cars), so there isn't much money to be made by selling a 9-year old BMW. And where I live, the depreciation on cars is lower than it is in most other countries. In your country, the depreciation is probably much higher.
