The use of the verb to be + verb-ing indicates the continuous aspect of the verb, i.e. the action takes place during a period of time in the present / past / future.
Then, there is a change in meaning in:
*where are the clouds that are supposed to be covering the city ? = at this moment
*where are the clouds that are supposed to cover the city? = usually, because it's the typical weather in this city.
As regards this sentence: he is a fine little piece to be selling himself for cheap on the street, I believe that the person who is speaking or writing thinks that he is such a nice person that he shouldn't be selling himself (prostituing?) on the street, but I would need to read the sentence in context to be sure.
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