Update to rules in trading forum.

[danielj]

This is a minor update, it just brings us in line with other message boards which facilitate trading:
We ask that people interested in items to state their interest within the thread where the item is made available, BEFORE sending a PM. We also ask that the seller/gifter pays heed to the order in which interest in items was expressed.

This prevents situations where people get gazumped and removes any ambiguity as to "who was first" (meaning that we don't have to wade in to adjudicate).

This will also be posted in the for sale forum.