Are sentences correct?

Properly speaking, "it" in your first sentence refers to the advertisement, not the job. Although all native speakers will understand what is meant, it might be better to replace "it" with "the vacancy" or a similar phrase.

Regarding your second sentence, I wish to clarify your employment of the phrase "used to". There is some ambiguity here!

Does the sentence mean

(a) This is the pen with which I was previously accustomed to write.

- or -

(b) This is the pen with which I wrote.

In either case the "with" is required. You write a letter, a note, a symphony, a novel, a cheque. You do not write a pen! However, people may tell you (wrongly, since the preposition has an object) that it is "wrong" to end a sentence with a preposition, and to please them you might be prudent to recast the sentence as at (a) or (b) above.

Thanks, Contrex, for your invaluable guidance.

Regards.