Hahahahahaha no. Most likely, you won't be able to manipulate the market consistently by yourself, even if you spent £10k / $10k.
The thing is that in order for deflation to happen, there needs to be a high supply of energy and low demand.

Supply < Demand = Prices go up.
We can clearly see this with energy peaking at about 9kcr, compared to when it was 6kcr.

When someone buys energy with real money, that is energy actually being made. It hasn't been used before and adds to the overall pool of total energy there is existing in Spiral Knights. However, because of OOO's somewhat discouraging promotions, less people are willing to spend actual money in order to consistently increase the total pool of energy a bit, every so often (much how like mints make and print money). As well as this, when people buy from the supply depot, that energy actually becomes non-existent. Once it goes to Kozma, no one is actually getting it (in a way, it is destroyed) and with that happening a lot, especially during flash sales, you can expect energy prices to skyrocket because there is a high demand for energy but there is less supply.

As Core said, you'll / OOO will need to make people be bothered to spend some money, so that there is more energy into the market and in the occurrence of supply > demand, people may want to sell their energy as soon as possible and so, lower their price of an energy listing by a few crowns. In order to consistently get energy prices to 5-6k, you'd need dozens of people who'd be willing to sell their energy for 5,000 - 6,000cr each and that somewhat seems horrendous, compared to 8,000 to 9,000cr when they're going to be making more money per 100ce and that's just how it is. With the abundance of crowns, I'm quite sure that even if people do wish to sell their energy at 5-6kcr, the price of energy will just go back to 7-8kcr because of the high demand of energy as it is the premium currency of this game and wanted by all.

ban rich players

That's not going to stop the market manipulator from putting up walls by putting up a cap for only a certain price. You will still see things like 9000 x500, 9001 x500, 9002 x500, all hidden behind a wall of 8995 x1, 8996 x2, 8997 x1, 8998 x4, and 8999 x1, to make players think they're legitimate sell offers.
Bundling prize boxes with energy again to entice players into buying Energy will help immensely.

I think that you should only be able to have up to 10 buy offers at a time, and 10 sell offers at a time. If you're actively on SK you can sort of make a wall, but it's not going to be a scary, unbreakable wall like x500 is.

On top of this put in a new rule about how you can't make price walls on alts. This makes borderline players less likely to do so, and players who keep it up can be reported or eventually banned from the game or from the market. I think a market ban would be more helpful however, as it keeps the player in the game, and now they can't generate CE from the market (and instead, must trade other players or buy the CE outright). Also keeps them on their main account so they won't go make more alts and do the same thing again without penalty.