I think the first issue here is the idea that these players are ‘lazy’. I mean we all are playing for enjoyment. This is not our livelihood. Next, these kind of players tend to spend a lot of their gaming time looking at trades. They try to search for bargains to come up, meaning someone who wants to shift something fast because they want coins fast. You may invest your time/what you enjoy doing into running quests, exploring, maybe gathering, whatever it is you choose to do to gain enjoyment from your play time. These market players enjoy watching the market, it brings them satisfaction to find a bargain, or to analyse where they can possibly make a profit.
That is not ‘lazy’, but just prioritizing their game time on something probably different to what you prioritise. It would not be my thing, ie I am not doing it.
Next, the so called real world is for example very different when it comes to harvest. There is a growing season, and after all the grain has been bought up, that’s it, no more grain until the next season. Now those who bought up the grain, can see where to set the price. If it was a good year for grain, there is more competition among traders and the prices tend to be lower. A bad season and grain goes up in price.
The game has no growing season. The nodes return over and over. Here it simply comes down to how many people are busy harvesting them, and how many players need/want what gets harvested. This can fluctuate. You could have 10 players harvesting happily all night, quit same time. Now there will be less nodes harvested, if other night players are not into gathering. This could create a shortfall and the demand may will rise above the supply. This however is not something a trader can foresee as such, although they could watch the change and now focus on buying whatever does not seem to be supplied as much, theoretically creating a bottleneck for the demand, but really only for a short time, because other players catch on as well, seeing there is a material suddenly selling for more, so they very possibly will start harvesting it to make money too.
You can have the developers announce a resource will be cut with a coming update. We saw this happening with refining, where the loot from boxes was reduced. Any player able to, not just traders, if smart, will have rushed to the trading posts and bought what they could while the prices were very low, because there had been more supply then demand for it. Certainly many traders will have done so, knowing they in the future would be getting higher prices for these now rarer materials, which regardless are always needed.
Traders are not lazy, they just invest their time differently. They can not control supply nor demand, they can only place educated guesses and watch and hope their guesses were correct. They tend to rather look for bargains, over buying all materials up, because there is no ‘end of season’. Your example with the salt is flawed because there will be lots of salt in the trading post, reason it dropped to 0.1. This means it is easy to come by, and even IF a trader would buy ALL salt, they can not stop the salt from still rolling in. They re-list their salt for the higher price you mentioned - and the next gatherer running into town will undercut, and then the next, and the next. Clearly there was more supply than demand, and so the prices will just drop and drop again. You can not create a bottleneck with items of that sort.
Now rare items, that’s a different matter. Where the demand totally outstrips the supply. Here a trader in theory could create a bottleneck, but it would mean such high investment, it’s usually not something that happens, simply because a trader would have to first amass insane amounts of wealth to buy all these rare items (still also being in competition with other traders, mind you, they regulate each other as well) and then hope and pray players will be willing to pay whatever the trader has to slap on top, because this game has taxes, so it’s not as clear cut as to say - I buy for 100 gold and that’s all I pay. They pay 100 gold and then tax, and then a tax for listing it again. That can get simply too much, and even with this - the next player rolling into town with such an item in their bag, will happily undercut them.
I think in these games, it is mostly about bargain hunting, and that takes a lot of time investment, so as already said, that’s not ‘lazy’.