Having more/unlimited GE buy/sell slots would improve matket liquidity in my view.
From my personal trading experience, wild price fluctuations are driven by traders' FOMO, as well as cycles of cash hoarding and bulk buying.
Take for example trading close to dxp weekends. Investors like me stockpile herblore/prayer items beforehand and then sell in bulk to make a profit. If we have dozens of different item stockpiles to sell quickly, but only 8 GE slots available, we'd sell at a price quite below the GE guide price, potentially contributing to the triggering of a market-wide panic sell. Flipping the coin, buyers may pour excessive GP into purchases in order to fill their orders quickly and move on to the next trading item, leading to a bubble. The Rune Index has been a prime example of this recently, smoke runes being one of them.
The markets for herbs, bones and potions have been 'pump & dump' markets for ages. Greater liquidity from more trade slots would help to alleviate this at least slightly in my opinion."Give me control of a nation's money and I care not who makes its laws." M.A Rothschild
I would support one or two slots being added in exchange for Loyalty Points(not coins).
My concern is adding in slots without any expense from the player would cause a uphill trend of monies to circulate. This increased circulation may be too much for the economy to handle(with a much lower player base).