I'm quite willing to apply for the sales representive position.
I have extraordinary interest in the sales representive position.
这样是最标准的, 说得天花乱坠是不切实际的. 这里翻译应该翻意思, 而不是单词翻译.
sales marketing position 没有这种说法.
或者是 sales, 或者是 marketing engineer/executive/officer
I would very much like to apply for the post of a sales representative of your company.
I am eagerly looking foward to applying for the the position of sales representative of your honour's corporation.
desired时态错.