but why not look for a smarter solution, a solution how don't affect the normal player ?
The problem is (correct me if I am wrong) that bot sent in the moment thousands of price updates every day, so why not either increase the possible price-update to 0.1 dec and/or take a few only from the 1001st lising in a month as 1000 listing will be enogh for a normal player but not for a bot.

I have also another solution: Take a few of 5 dec for every price change, but let it free to list a card and also to take a card away from the market. To prevent bots from missuse just implement a cool-down-time (f.e. 1 hour) where a card who was removed from the market cannot be listed again.

How about this ?

Than just I want check your point: "Discharge is going to be one DEC or 1 credit which is very minimal."

Really ? Is it always in that way ?
A lot of cards are listed now for less than 1 dec / day for rent.
But we can also check about to sell cards.
Let's say you open a Chaos Leggion Pack and you get some common cards who you have already at max-level, so you want sell them.
The price for common Chaos Leggion Cards is just a little bit over 0.02$, so let's say you want sell them quick for this 0.02$ than you pay 0.001$ for listing and 0.001$ as commision (5%), so from 0.02$ Splinterlands take 10% or 0.002$ - before it was 5% so they just double the price. Is this really minimal ?