the price of a commodity depends if i sell it or if i buy it

no that's wrong.

its a schoolboy error you made there, this is the nonsense they teach you in school about business studies.

in the real word, what dictates price is how much stock you have of an item left.

the premise of business is that you have something to sell, nothing to sell equals no business.

so what happens is as the stocks drop the business put up the price to ensure they don't run out of stock.

its retarded in the long run, but its the way its done.

You should take two of these and call me in the morning, son.

You should be a market maker and not a market taker if you want to make money trading.

You're paying the spread instead of receiving it.

You can't make money lifting offers.

If you don't understand what I just said, stop trading now.

i was thinking about an hypotetical real case

You need to talk to GLP member Donny Boy. He's an economics genius!