I think it's actually the other way around. If you don't see trades for a good it's usually because they get picked up quickly. As a scrolls boosted player from before the moonstone library fix, it's the same tier trades that sit in the trader that are a more accurate guage of what goods are hard to move. At this point though, I'm not seeing any same tier trades sit for long. Steel is maybe a little more scarce, but even my trades for that get picked up in a few hours at the most.
This is one of the problems with making generalizations about trade based upon what you see in your trader. My wife's fellowship, she tells me, never trades, but when she posts trades they take them immediately. What? If they never trade, I suggested, why do they take your trade post haste? "Because I'm me and everybody likes me," she replied. I had to agree. LOL. In reality her fellowship is so good she never sees trades because they ALL go quickly. So they are masters at trading. My fs is the same way. I seldom see many trades because they disappear almost instantly.
So in the end the same principle applies and you might be right. Marble just goes faster than every thing else.
On another note, whatever imbalances appear they will generally work themselves out as things progress. All the gems for T1/T2 trades I saw a few weeks ago have become less and less as those trying to dump gems have cut back on production, dumped their excess and/or built T1 and T2 production to balance their cites more. That's pretty much how it works.
And finally, another thought. If you take a trade that is offering a good that is in short supply at a 2 star rate for something at the same tier that is over abundant, you are shorting yourself. 2 star assumes all goods on the same tier are of equal value. Supply/demand says otherwise.
Supply/demand would suggest you make the trade more balanced by adjusting for the supply/demand of each good. In other words, you'd avoid 2 star trades between scarce goods and abundant goods since 2 star would not reflect what you could get for your offer of scarce goods, the true "market value" of the goods being offered, before the intangible values are added by the actual trader. And if you take such trades -- someone offering scarce goods for abundant ones -- you perpetuate the problem because there is not incentive to balance their production exactly because they are being told that a 2 star trade is always equitable and if they don't question this (and most don't) they keep offering their scarce goods for less than they could/should. All of which slows their growth, and hinders markets adjusting to supply/demand issues. It's no wonder the imbalances we experience take so long to adjust themselves.
AJ