The thing with trading in players is that it's not like going to a nice retail store where you just waltz in and order exactly what design you want, at the exact size, for the exact price you want to pay.
I imagine its really more like a Sunday morning trash and treasure stall. Youre not really sure what you're going to find, and you have to sort through what's there to find what fits - sometimes there might be something you weren't really looking for or don't even really need, but you can't ignore the quality. Or someone's junk might be you're treasure, you just have a bit more imagination.
So maybe a 185cm mid might be perfect, but maybe a 175cm mid is all that's sitting on the table. As long as they work, does it really matter?
It's really about what's there, what's available, and what you can make of it.