Hello
My Dad has a 1980 633 CSI, picture below, with 90k miles on it. He is original owner and wants to know if it's worth selling and what a fair price is.
Any help, or point in the right direction will be helpful!
Thanks

http://www.motorheadcity.com/images/dads2.jpg

Not sure what you mean by "is it worth selling?" question. But the car looks pretty nice which is a plus, but of course the fair value also depends on the mechanical condition of the car.

You might try browsing a few web site to figure up what it might be worth:

roadfly.org
bimmerbrothers.net
autotrader.com

analogy:
house : location, location, location :: 6 series : Mileage, Mileage, Mileage

This car is worth a bit being the original owner and such low mileage. However being a 633, its not so high in demand.
I would keep it.

Unfortunately - Joe is basically right -

The low mileage is a plus - but - even with low mileage - the mechanical and cosmetic condition of the car will determine to overall value.

if its in perfect shape - i.e. no cracks in dash; perfect seats; perfect paint; all electrical critters well - etc; and the engine bay is nice and pristine; he has all the records of maintenance and repair/replacement - etc. then he may be able to EEK 6-8K out of it from a willing buyer.. (that's holding out basically - for someone who loves the 6ers (like me :) ) - that has money to burn (not me :( )

if it needs stuff - like it's been sitting for a few years cause dad hasn't driven it; etc - paint has chips; oxidation and scratches (it doesn't look like it from the pic) then dad will be lucky to get 2 grand out of it cause it'll cost 4-5 grand to get it back up to snuff.

the downside to that car is that since it's a 1980 car - it can't be chipped; and it runs the older Bosch injection and engine management (why it can't be chipped) - but they can be turboed from what I understand.

also - being 1980 model - it's based on an E12 chassis and not an E28 chassis - easier to work on suspension; not as many electrical bits to go wrong and work on (i.e. no abs; airbag etc.)