Your best bet would be looking at kbb.com
I looked it up for you. Your car has a value of $39500 Private Party sale and $37000 Trade in.
So I would say the dealer is offering a good deal. You can get little more with Private sale of course.
2011 535i Alpine White, Oyster on black interior, Premium 1 & 2 Package, Magnaflow Full Cat-Back Exhaust, JB4 Stage 2