I was also curious about that, thanks for the explanation!
Am not interested in buying or selling from other members but I like to be informed on all possibilities!
There is another thing to remember: you can only sell "contracts". If you've 100 MH/s Scrypt which you bought in 1 time, it's a contract of 100 MH/s so you can only sell it completely.
But if you bought this hashrate in parts, for example you choose for reinvesting, then you can have 100 contracts of 1 MH/s each, and then you could sell them seperately.