Analogy was between market making (buying and selling the item in Runescape with a spread) and second hand book sellers (having a buy/sell price that's not equal to each other for a second hand book). Citadel Securities does not do naked shorting and does not fail to deliver - they make markets by taking position both on long and short side of the market with a spread in between. Some of the short positions they take may not have an immediate backing - but they make up for that by offering more more and attractive prices for bids making up the shortfall within a matter of milliseconds.
Sorry, I fail to see the analogy.