He works in a bookstore and Christians come to buy Mushafs to give as gifts to Muslims; can they touch them?

Firstly: 

It is not permissible for one who is in a state of impurity
to touch the Qur’an, whether that impurity is major or minor, because of
what is mentioned in the letter of ‘Amr ibn Hazm that the Prophet (blessings
and peace of Allah be upon him) sent to the people of Yemen, in which it
says: “No one should touch the Qur’an but one who is in a state of purity.”
Narrated by Maalik, 468; Ibn Hibbaan, 793; al-Bayhaqi, 1/87 

Al-Haafiz Ibn Hajar said: This hadeeth about the letter
mentioned was classed as saheeh by a number of leading scholars, not in
terms of the isnaad, but in terms of being very well known. Ash-Shaafa‘i
said in ar-Risaalah: They did not accept this hadeeth until it was
proven to them that it was the letter of the Messenger of Allah (blessings
and peace of Allah be upon him). 

Ibn ‘Abd al-Barr said: This letter is well-known to scholars
of the Prophet’s biography (seerah), and its contents are well known to the
scholars. It is so well known that there is no need for an isnaad, because
it is akin to tawaatur (the process by which a hadeeth or report was
narrated from so many by so many that it is inconceivable that they could
all have agreed upon a lie), so it was accepted by the scholars.

End quote from at-Talkhees al-Habeer, 4/17 

This hadeeth was classed as saheeh by Shaykh al-Albaani in
Irwa’ al-Ghaleel, 1/158 

The kaafir (disbeliever) combines both kinds of impurity: if
he becomes junub (impure following sexual activity), he does not do ghusl,
and even if he did do ghusl, it would not be valid. Hence the majority of
fuqaha’ are of the view that the kaafir should not be allowed to touch the
Mushaf. 

It says in al-Mawsoo‘ah al-Fiqhiyyah, 38/19: 

Kaafir touching the Mushaf or working in the copying out or
production of Mushafs: 

The kaafir should be prevented from touching the Mushaf, just
as a Muslim who is in a state of janaabah should be prevented from touching
it. In fact it is more important that the kaafir be prevented from touching
it at all, whether he does ghusl or not. In al-Fataawa al-Hindiyyah
it says that Abu Haneefah said: If he does ghusl, it is permissible for him
to touch it. And it was narrated in al-Bahr from Abu Haneefah and Abu
Yoosuf that he should be prevented in all cases, and the kaafir should be
prevented from working in the production of Mushafs. Similar to that is what
al-Qalyoobi said: The kaafir should not be allowed to bind the Mushaf or
apply gold colour to the cover… End quote. 

See also 37/282 

The majority of fuqaha’ are of the view that the kaafir
should be prevented from touching the Mushaf, because the kaafir is najis
(impure), so the Mushaf should be kept away from his touch. 

Muhammad, one of the companions of Abu Haneefah, held a
different opinion. He said: There is nothing wrong with him (the kaafir)
touching the Mushaf if he has done ghusl, because the factor that is
preventing him from doing that is physical impurity (hadath), which is
removed by doing ghusl; rather what remains is the spiritual impurity of his
belief, but that is in his heart, not in his hand. End quote. 

See also the answer to questions no.
100228 and
96646

Secondly: 

It is permissible for a kaafir and one who is in a state of
minor impurity to touch a tafseer (commentary), a translation of the
meanings of the Qur’an, or a book that contains the Qur’anic text,
translation and commentary. 

Shaykh Ibn Baaz (may Allah have mercy on him) said: With
regard to the translation of the meanings of the Qur’an, there is nothing
wrong with a kaafir touching it, because what is meant by a translation is
that it is a book of tafseer and is not Qur’an. In other words, the
translation is a commentary on the meanings of the Qur’an, so if a kaafir or
one who is not in a state of purity touches it, there is nothing wrong with
that, because it does not come under the rulings on Qur’an. The rulings on
Qur’an only apply to that which is written in Arabic only and in which there
is no commentary. But if there is a translation with it, it comes under the
ruling on tafseer (commentary), and it is permissible for a person who is in
a state of minor impurity or a kaafir to carry it, because it is not a book
of Qur’an; rather it is regarded as a book of tafseer.

End quote from Fataawa ash-Shaykh Ibn Baaz, 24/340 

See also the answer to question no.
10694 

Based on that, it is possible to be gentle and polite in
preventing the kaafir from touching the Mushaf by hastening to ask him, “Do
you have wudoo’?” That is so as to inform him that the Mushaf can only be
touched by one who has wudoo’. If you yourself do not have wudoo’, and you
pick up the Mushaf with a piece of cloth, or you put on gloves, for example,
to open it and turn its pages, this will be something good that will
demonstrate the sanctity of the Qur’an and will dispel any doubt from the
Christian’s mind, because we prevent a Muslim who does not have wudoo’ from
touching the Mushaf just as we also prevent the non-Muslim from doing so. 

If he wants to buy it, you can pick up the Mushaf and show it
to him, and give him something to put on his hand if he wants to look at it
himself, then you can wrap it for him in a wrapper that is suitable for
giving. 

It is permissible to give him in the first place a Mushaf
that has a commentary in the margins, or a translation of the meanings; in
this case it is not required to be in a state of purity in order to touch
it, as stated above. 

You should also try to wrap it for him, so long as you know
that he wants to buy it to give as a gift, to prevent him from touching it. 

And Allah knows best.