"He should sell computers, he is good at it."
Translation:Powinien sprzedawać komputery, jest w tym dobry.
Not really, it would be weird ("He is good at it" = "Jest w tym dobry"). But the word order suggested by you could be used in other sentences: "Jest dobry w tym, co robi" = "He is good at what he is doing" (literally: "He is good at that, what he is doing").
Too many errors like that: I wrote "On powinien sprzedawać komputery, on jest dobry w tym." - And wrong :(. Please add a variant.
That really doesn't sound natural. First of all, the repetition of "on" is maybe not incorrect but at least not natural, and the word order in the second clause - I don't think a native would say that.
Why is there no on for he? I always hear it used as a kid and it messes my mid\nd up now.