Basic rule of car buying. If in doubt walk away regardless of the deal. For prices check out auto trader, pistonheads and other on line publications. Try and find similar mileage and specified cars. this will give you your price. For example if you can find the same spec and mileage at a BMW dealer this car has to be worth £1500 less. Non main dealer then £750 less as a guide price. I don't mean to teach you to suck eggs as i am sure you know what you are doing.
With private sales i find that they often have unrealistic valuations and expectations of value. For example i bought an E60 530d for my father and we found a Private sale with 29k for £17k Found a dealer in manchester with a 2 year newer car 31k for £16,500. I offered £15k and they would not accept anything less than £17k. So we went to Manchester and bought that car for £16,250. 3 months later the £17k car was still for sale. In other words you can be a good haggler, but that does not mean you will get the deal at the price you want.
E90 330i SE Auto Graphite Grey Metalic