Well it comes down to the fact that a lot these players we loan out and eventually sell are actually good or at least very decent players, and by loaning them out they have the chance to at least prove themselves to some extent. So if they end up not being Chelsea quality, which is pretty much always the case, they are still good enough for mid table clubs who are willing to pay, what is for them, top dollar for these players. To top it off we originally payed rather small prices for them too, so it's almost always a good profit. Then our purchasing has been pretty decent too, not always, but because we're able to make good cash the prices we pay for some of the guys we buy doesn't seem as bad.
That said, I really can't explain Schurrle. If anything his value should have gone down, the only thing I can think of is that he won the WC and played pretty well there. He's not a bad player but he's infuriatingly inconsistent. Can't really explain the Luiz deal either. I guess it's down to the buying club, if we know they have money, like PSG, we know how to wring as much as we can out of them. We're effectively doing to all these clubs what they've been doing to us for years.