Week 3: Market Design and Price Discovery

Order books, guide prices, and why Old School RuneScape's Grand Exchange and a stock exchange are closer cousins than either fandom would like.

8 March 2027

A market needs a way to turn “I’ll sell for at least X” and “I’ll buy for at most Y” into an actual trade, and different games have made genuinely different choices here.

Two shapes of market#

Matched-order books (RuneScape’s Grand Exchange): players post buy and sell offers into a shared queue, and the system matches them automatically at a clearing price. Nobody meets anybody. The published “guide price” is a lagging average of recent trades, not a live order-book snapshot — which means the number on screen and the number you’ll actually pay can quietly diverge, especially for anything traded rarely.

Listing markets (a WoW-style auction house): sellers post a fixed asking price, buyers browse and buy at that price or don’t. There’s no matching engine deciding a clearing price — the “market price” is just whatever the crowd of current listings happens to say, which makes it easier to manipulate with a handful of coordinated accounts.

Why the Grand Exchange enforces a price band#

The Grand Exchange caps how far an offer can sit from its guide price. That isn’t a courtesy — it’s a scar. A market with no band is trivially cornered: buy up the visible supply, then relist it at ten times the price to anyone desperate enough. The band doesn’t eliminate manipulation, it just raises the capital required to move the price meaningfully, which is the same tool a real securities exchange’s circuit breaker is doing, aimed at a different failure mode. There’s a general theory for why a rule like this is the “correct” fix rather than just a plausible one — Week 11 names it properly.

Studio this week#

Market Data Lab: you’ll get a real (anonymised) slice of trade data and be asked to find where price and guide-price diverge, and why.