KBB leans notoriously high on its estimate of what sales prices are achievable in most cases
KBB has been notoriously low, in my experience. The last three cars I bought couldn't be found for less than $1000+ higher than the KBB price. And even then, when I told other car enthusiasts about the price I paid, they were impressed I had gotten such a great deal. Of course, that's because I buy weird cars and am picky about things like manual transmissions....
However, I just checked Autotrader and that KBB does appear to be accurate for a 2007 335xi.
Remember, you are looking at
asking prices on Autotrader, et al and most everywhere else you look. No doubt some of those will not budge downward a penny even when money is waved over them, but a lot will.
Ideally, genuine auctions would tend to establish what a cash-equipped and informed buyer is actually willing to pay for a given car today, but getting a bead on comparables from auctions is a trouble fraught affair. I don't recommend it except in pure theory.
No point contending over that, though. You are right; you are always right. -But still- OP could become rich using KBB (or Autotrader, etc.) as his absolute buying bible and using his super-spidey selling powers to turn twenty five percent a month on his involved capital,
and drive free (except gas) while he flips units at a couple thou over the bible-ordained prices, nicht wahr Schatz?