Questions should be public because they can be interesting to other people. It should be avoided that different users ask the same questions.
Interest should be public so a potential buyer can see if he can expect a success or if there are many other interested persons and he probably won't get the item.
No. A third person should not link two sellers to each other. He should only answer if he is interested, but don't want to pay the offered, but a lower price.
A third person can answer in "wanted" topics to link to "sell" offers if the seller did not do it.