Partial file sharing via HASH and have a option to not allow a complete file or say only 20% to reside on your system at any one time and

If high speed people would simply let others share THROUGH their node, then it would be very difficult to say you are 100% sure it's on someone's computer.
Even if people never use the pass through feature, since you can't tell if someone is using it or not, then there is still no 100% way to know where it's coming from.
You also need a way to block IPs from passing through, let's say you only allow certian IP groups because you have nothing better to do.
Why do that? because you could use another client and try to connect & pass your files through, and if it doesn't happen then you can say "that guy isn't passing through". But maybe, just maybe that guy is only passing certian IPs he likes! So no 100%.

Oh, and if you say "that would make trading too slow", you haven't been paying attention.