I'd have to say 3 and 4 are grammatically correct, but none of them sound too pretty. I'm sure there are better ways to put'em, such as "I bought / sold it at a good price." Oops, I just realized buying something at a good price holds the opposite meaning to buying something expensively. In that case I'd just use a different verb: "I paid too much for it"
I can see where 1 and 2 are coming from though, as I often say "I bought it cheap", although I guess "I bought it cheaply" would be the right way to say it.
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