I believe it's a decent idea but needs refinement: People would only move their things to cheap RS gold be at the top end of the 10 percent, there by pushing the average GE cost, rinse and repeat and also the price of the item on that day will likely bloat very aggressively. If issue 1 is compared to wealthy players wait patiently to market on the day and will stock pile items, as it is the price and an payout could be manipulated. I believe that there needs to be a lockdown about this thing of the day's purchase price, prior to it.
Possibly a different mechanic may be used, like the thing of the afternoon is worth 1.1X it's average GE value in gold if you fall it directly in the pit. In that instance it's 10% less costly on average to throw the items in for the pet. I assume this could be abused the. Could address it by not. There's zero reason for us understanding it other than to abuse that, so just don't tell us. I'm sure some folks may figure it out, but they would be guessing, and they could also change up the item to 2-3 objects rotating during the day or any number of solutions to deal with people abusing it.
I meant like once a product is chosen we'd know almost instantly, not beforehand (you could attempt to predict ahead of time but that may not be accurate like you said). How I read the layout of OP there will be a number of those'item' after it's chosen, removed from the market. So this huge spike in item sells and/or related change could be an alarm bell that is instantaneous. Any somewhat effectively trained algorithm would have the ability to decide on these sink things up (assuming they were really having an effect on the market, which if they are not, then what's the purpose ). With the method I'm describing it would not be something you could call ahead of time but it would be something once it occurred, that you would know fairly fast.
More broadly, I'm simply pointing out that its very tough to'hide' an artificial constraint (at least one which produces a significant impact) on a marketplace if there is sufficient data readily available on the market. Demand curves and product prices are quite showing on things like that. By having a list of items instead of just one that are selected or intentionally jumped around, like obfuscating there might be workarounds. However, it would be a problem for OP's design if implemented word for word.
Make it an occurance that occurs in the end of the dump that is gold. If ultimately 10billion is tossed the GE sales of the unknown thing are subsequently bought out. No telling of it happening at all. No chance for abuse. Yeah thats not a bad idea. To minimize abuse potential even further I would also suggest making the buyouts happen at random intervals, to stop people from only stockpiling random items and then posting them at 9.9percent over market place before the buyout occurs. So then if the buyouts are arbitrary its still possible to do so, but you'd be sacrificing all of your GE slots to buy rs 3 gold have things sitting there all the time so it would be a bet.