In compound structures, words are often left out for economy: Tom is a man who means what he says and [who] says what he means. Such omissions are acceptable as long as the omitted words are common to both parts of the compound structure.

If a sentence defies grammar or idiom because an omitted word is not common to both parts of the compound structure, the simplest solution is to put the word back in.

The word who must be included because whom . . . have purchased is not grammatically correct.

Has . . . accept is not grammatically correct.

Believe . . . by is not idiomatic in English. (For a list of common idioms, see 18d.)

note: Even when the omitted word is common to both parts of the compound structure, occasionally it must be inserted to avoid ambiguity.