It's not a small impact.  I have destroyed the market value on a lot of items.  Others do this as well.  You simply save up months worth of stock from a worker empire.  This may also include gatherable items or farmed items, but mainly the worker proceedes.

You then activate a month long VP.  The goal is to sell 500,000+ of a standard item.  In the first few hours/days they might sell decently and get a near max value.  However, what this does it stiffle all the other sellers.  People start seeing they are not selling much or any of their items and the road to bottoming out the price begins.

So in order to compete and reach the goal all the orders start going down quickly.  This will continue all month until everyone is trying to sell the item at min price.  It's an artifical restriction causing this issue.

Eventually, the stock will run out or the VP will expire.  However, this is the damage from a single player.  When you tally up all the players in this position the overall effect is much worse.

It's also easy to tank the prices of highly valued items.  This isn't THEORY, I have done it on LIVE SERVERS many times and will continue doing it.

If you have a that fat stack, then you didn't sold for like a year or two while your workers work 24/7...

That is correct and a common practice.  With the scarcity and cost of value packs many players go week, months, even years before activating them.  This can also occur if you are undecided on the item and hold onto them trying to make a decision.

The number also widely varies depending on the resource.  Some have much higher yields or can be obtained via multiple nodes.

Is it common practise to lecture everyone about how bad are AFK features supposed to be while secretly hoarding half a million of a single material through running your computer 24/7 for years.

For Retteo/Vortexii/2Cents this kind of massive hypocrisy and contradiction is correct and common practise.

This situation doesn't exist, except artificially in peoples heads. 

If you are playing on a limited VP FTP account you would never do this.   You'd post all your sales normally, and you wouldn't collect on them... not until you pop a 1D VP when you decide to collect.  Rinse, repeat the same rotation of loyalties to get a 1D VP.  

Even if you are on the last 3 days of a 30 day expiring, there is absolutely no reason to do it, and just do the above.  If you aren't you are just shooting yourself in the foot. 

This is another of your completely made up scenarios that don't exist.  

You can only list a specific amount (e.g 5k elixirs) at once. This is a common practice that the OP is describing and almost everybody who uses 1 day VP do this and the result is always the same. A massive decrease in value for specific items.

If there was not such a massiv tax or everybody would use VP all day long you would not see such cheap prices for a bunch of stuff.