How to sell items in flea market instantly and despite price of the offer?

It looks like Aki Configurator has the options you're looking for.

Its in the readme

so set chance to 100 and timer to 0 (or 0.1 or something maybe)

Make sure you have the latest version

For the speed that is time. (in minutes by the look of it)

Try with

"chance": {
    "base": 100,
    "overpriced": 0,
    "underpriced": 0
},
"time": {
    "base": 1,
    "max": 1,
    "min": 1
},

Or with over/under at 100 maybe?

First, have you made sure to close out of the server and use the launcher to clear temp files (on the settings tab) between changes? If you haven't been doing that, that might be at least part of your issue.

From there, after looking at the at the config, I think the default values mean:

Base: 50% chance to sell when price is near the "average" price.

Overpriced: chance to sell = base * .25 when priced above the "average" price.

Underpriced: chance to sell = base * 2 when priced below the "average" price.

So setting the price-based modifiers to 0 is likely going to make sure that absolutely nothing you list will ever sell, no matter how low you list it for. Try leaving base at 50, and changing the two modifiers to some arbitrary whole number (e.g. 5).

Also, given that the time to sell seems to vary, it's likely that sales work as a dice roll that the server periodically recalculates. With that in mind, I'd start by leaving base and max alone, and only lowering min to 1.