Personally, I don't think the Grand Exchange should be brought to Old School. I liked the Grand Exchange, it was quick and easy way to get items and fulfill trades. However I like what we have with the trading post, I think the you should aim to expand this instead. The trading post is great because you can actually see the offers people have posted, I like that aspect of it. You can see what the lowest price is that people are selling items for, you can see that they are online, with items in stock and you can buy them; simple.
Ideas for expansion of the trading post:
Cross world/Map trades.
I do think there should be the option to trade without having to meet the other player. This in some instances would be great, say you put an item up for sale and no one buys it. You get fed up of waiting and travel all the way to Yaks. We've all been in the situation when you only just arrive somewhere and you get a pm asking if you're still selling said item. In a situation like that, it would be great to be able to nip into the bank, open up the trading post and collect your money/items. Also if this is the case, players with private chat turned to 'friends' or 'off', could have their names highlighted a different colour. Therefor upon seeing the offer you know that person is only trading through the Cross World/Map trading.
Average Price.
I believe an average price chart would be a great addition to the trading post. This is another part of grand exchange people like, and I totally agree it comes in very useful.
Price checker.
This goes along with the previous suggestion of the average price, and the price checker suggestion you have put into the Dev Blog.
These additions have previously been mentioned, by other players and yourselves working at Jagex. However a lot of people seem to be voting yes before the poll has even started! I just feel that a lot of OS players came here for the nostalgia, and slowly, with ideas like the Grand Exchange... We're losing that.
12-Jan-2015 14:45:36