oh boy ...
RU: Ja prodal jemu piesok.
Is this an expression in Polish?
Nothing idiomatic. "sprzedać piasek na pustyni" (to sell sand in the desert) is idiomatic.
Like in English "selling ice to an Eskimo"
"sprzedać lód eskimosom". Also works :)
or "eskimosowi" if you really want one Eskimo, but I prefer the plural version.
The most popular English idiom would be "Selling, carrying, bringing, or taking coal(s) to Newcastle!" :)
some seems more likely than the