Something that has a known price, one may not sell it above market price on the basis that he is going to have to wait to receive the mone. However, if it doesn’t have a known price, even though if [the buyer] were paying now he would pay less and it’s because he has to wait for the money he charges him slightly more, it is permitted. But only if he doesn’t raise [the price] a lot, which is described in the Chavos Daas as being 1/6 or more, until it’s recognizable to all that because of waiting for the money he increased the pay. Also, even if he does not raise [the price] alot, but he explicates and says “If you give me immediately the money, you can have it for 10, and if it’s delayed, give me 11” it is prohibited.
Similarly, if he buys the item for much money in order to immediately sell it and lose — just to have the money in his hands for some time, this too is prohibited.
Interest on late payment that is smaller than the minimum prohibited as ona’ah (price gouging) can be rolled into the price of the item, as long as it’s not explicit that that’s what is being done.
The last line seems to me to be more similar to the commodities trader. In that case, the person lent the item in order to be repaid in the same item once it’s worth more. Paralleling the lender in the case of normal interest. Here, the person is willing to take a loss, much like the borrower in a normal interest case.