Seeing the only reason this car is 28k is because of the mileage, but the fact that it's been modded probably means it's been thrashed abit, and it's still 10 years old - and the mileage isn't so important, I can't see the justification in that price.
I paid $13k for a 70,000km 2008 335i almost a year ago. Completely mint with all options. It would never have given me any issues if I hadn't had tuned it.
BUT, as soon as you wind the boost up, thing's start to show their weakness. 30-70,000km is when things will fail under the higher pressure.
TBH I just can't see how this car is worth more than 17-18k