Question:
How we will trade less common goods through completely blind GE? Should we post on forums announcements like "selling 30 fish potions(4) 400ea in Grand Exchange..."?

Overview:
There is one big problem with actual Grand Exchange, inability to find out what offers are currently posted in system. Without at least this information players are unable to decide right prices and form of commodity in offer. It will be however much better if it will be possible to see more statistical information to be able to decide if it is worth to cast offer to GE at all or not. Number of possible offers is limited and GE is unfunctional for less common goods becasue of this blindess.

Suggestion:
To solve this problem we need to know how many offers (and of what type) are actually in system. Just global numbers. Even basic global number like "global last know balance of offers" = "total quantity of last known sale offers" - "total quantity of last known purchase offers" will be really big help. There are two possibilities how to implement this to game interface:

Simple global number can be displayed after names of found commodities by search engine of purchasing mode. This will allow players to check "last known" availability of goods. Actually it is really hard to sell less common forms of goods through GE. And because number of GE offers per player is limited, sellers tends to not give sell offers for hard sellable stuff to this system. The information about known offer or demand can help a lot with this problem.

Another solution is to display this number somewhere inside purchasing order interface after selection of goods. But it should to include global numbers of related goods, eventually with ability to switch between forms without going back over whole engine searching. For example, if I want to buy some potions, I usually choose most common 3-doses potion. But if I will get information that there is actually suitable offer of 4-doses potions I might wish to buy them instead. Especially if I am already stocking this sort of potions in 4-dose form in my bank.

It might be also useful to have more information about commodity trades. May be through button in offer interface. Data like actual "last known" sale and purchase offers, offers made during last day/week, Trades made during last day/week (all of this just as global quantities) and graph of average price changes during last time period. This will help players to find "trends" and also to make idea how hard it will be to sell/purchase uncommon stuff. However, this information is optional and we can live without it.

I am using term "last known" instead of "actual" because retrieving of actual online data and/or pushing them to users might negatively affects performance. It depends on system implementation and it is unknown to me. But I think that it is easily possible to let "trading" server to periodically update special commodity table used by search engine with this information and this table can be then used to display it to users searching for goods or opening offer interface. The update period should be pretty short, I think it may not exceed 2 minutes. Statistic information can build on on-demand basis directly from precomputed datas in database, there is no problem with longer response times. Basic data for this statistic info should be computed on idles, probably hourly and daily.