I know many of the shoppers are not going to like this, but I'm going to put it out here anyway.
I find it odd that that some people will pay 41k for pajamas that make you bounce, but are not willing to pay a decent price for other items. Example: "I buy black marble 15/1c". Here's the problem with that. In a forest mine, the most plentiful block is dirt; grass only covers the surface; trees and bushes are not on every Grass Block so they are less plentiful; and marble has to be dug out of the ground. The buyer needs to take into consideration that the seller has gone to the trouble of traveling to the forest mine and digging around for marble. I have found that any one section of mine with marble will give you only 16-20 blocks so that makes them fairly rare. If you are too lazy to go out and look for the marble yourself, then you need to pay the seller for the labor involved in getting it for you. Don't expect the seller to give things away.
In addition here's a little lesson about retail and wholesale. Retail is the price you pay when you buy, say, a sweater from the clothing store. It is meant to be the FINAL price on the sweater and for you to wear the sweater yourself (or give it as a gift to someone who will wear it themself). Wholesale is the price that the clothing store pays for sweaters in bulk amounts so they can stock their shelves. They buy at a lower price because they are buying lots of the same sweater at a time. Then they take that price per sweater and mark it up about 300% so they make a profit when you buy the sweater in their store. This is the reason so many buyers want rates "cheap". Why would you expect someone to pay 350c retail to get a hat in the cubit store and sell it to you for 50c wholesale? That makes no sense. It seems that the chances of getting an expensive rare item from dehydrated cubes is very small. Again, don't expect the seller to give things at whole sale prices unless you are buying in bulk (like thousands of items).
If you need cubits to buy the things you want, get out there and mine. Yes it's a slow process, you only earn one cubit for about every 16 blocks pulled. But if you stay with it you can earn enough, and it won't take that long. Stop worrying about being rich, looking cool and impressing your friends with the latest new thing. It's not worth the stress.