Is it permissible for him to complete the purchase when he can hear the call to prayer?
Question

                Sometimes when I'm in the market place and I am in a store and have chosen something and am at the checkout, the call to prayer is given. Is it permissible for me to complete the purchase or is that haraam? 

Also, sometimes we are in the car and the muezzin has already given the call to prayer, and we stop at the grocery and my brother asks the shopkeeper to sell him something. Does this also come under the prohibition?.
Praise be to Allah.
      If that is the adhaan for Jumu‘ah prayer that comes after 
      the khateeb has ascended the minbar, then it is haraam to buy and sell at 
      that time, because Allah says (interpretation of the meaning): “O 
      you who believe (Muslims)! When the call is proclaimed for the Salaah 
      (prayer) on Friday (Jumu‘ah prayer), come to the remembrance of Allaah 
      [Jumu‘ah religious talk (Khutbah) and Salaah (prayer)] and leave off 
      business (and every other thing). That is better for you if you did but 
      know!” [al-Jumu‘ah 62:9]. 

    The words “O you who 
    believe (Muslims)! When the call is proclaimed for the Salaah (prayer) on 
    Friday (Jumu‘ah prayer), come to the remembrance of Allaah [Jumu‘ah 
    religious talk (Khutbah) and Salaah (prayer)] and leave off business” 
    mean: stop dealing with trade and come and listen to the khutbah and perform 
    Jumu‘ah prayer in the mosque with the imam. This means that it is haraam to 
    buy and sell after the second adhaan which comes when the khateeb sits on 
    the minbar, until the prayer ends, unless there is a case of necessity which 
    calls for buying or selling, such as buying water for the purpose of 
    purification or a garment to cover one's ‘awrah for prayer. End quote. 

    Fataawa al-Lajnah al-Daa’imah, 
    13/101-102 

    Shaykh Ibn ‘Uthaymeen (may Allah have mercy on him) said: 

    Buying and selling after the second call to Jumu‘ah is haraam 
    and also invalid. Based on that, the transaction is invalid, so it is not 
    permissible for the purchaser to dispose of what he has bought, because he 
    has not taken possession of it, and it is not permissible for the seller to 
    dispose of the money paid, because he has not taken possession of it. This 
    is a serious matter, because some people may buy or sell after the second 
    call to Jumu‘ah, then take things on the assumption that they now belong to 
    them. End quote. 

    Al-Sharh al-Mumti‘, 8/52. 

    So it is not permissible to buy or sell after the second 
    adhaan for Jumu‘ah, because what is required is to hasten to the mosque in 
    order to hear the khutbah and pray, and buying and selling distracts from 
    that. 

    But if that happens with regard to all the other prayers 
    (Fajr, Zuhr, ‘Asr, Maghrib and ‘Isha’), there is nothing wrong with it, so 
    long as it does not distract one from praying in congregation and cause one 
    to miss it. 

    Allah says (interpretation of the meaning): “Men whom 
    neither trade nor sale (business) diverts from the remembrance of Allaah 
    (with heart and tongue) nor from performing As‑Salât (Iqâmat‑as‑Salât) nor 
    from giving the Zakât. They fear a Day when hearts and eyes will be 
    overturned (out of the horror of the torment of the Day of Resurrection)” 
    [al-Noor 24:37]. 

    Those are men who, even if they do business and sell and buy, 
    there is nothing haraam in that, but it should not distract them and make 
    them give precedence to that and prefer it over the remembrance of Allah, 
    establishing prayer and giving zakaah; rather they should make obedience to 
    Allah and worship of Him their ultimate aim, so whatever keeps them from 
    that, they should reject it.

    End quote from Tafseer al-Sa‘di, p. 569 

    Shaykh Ibn Baaz (may Allah have mercy on him) was asked: Is 
    it haraam to buy and sell during the adhaan or after it? What about on 
    Fridays? 

    He replied: 

    Allah says (interpretation of the meaning): “O 
    you who believe (Muslims)! When the call is proclaimed for the Salaah 
    (prayer) on Friday (Jumu‘ah prayer)…” [al-Jumu‘ah 62:9], so it is 
    not permissible to buy and sell or to rent or do any other kind of 
    transaction; rather people should focus on Jumu‘ah prayer and hasten to pray 
    Jumu‘ah, and not let anything else distract them. 

    With regard to other prayers, they may or may not come under 
    the same rulings as Jumu‘ah, but to be on the safe side, one should not do 
    anything after the call to prayer, after the adhaan for Zuhr, ‘Asr or 
    Maghrib, because that may distract him from praying in congregation. So it 
    is better to avoid that, unless it is some minor matter that will not 
    distract him, so perhaps there is nothing wrong with it, because Allah, may 
    He be glorified and exalted, only spoke about the issue of Jumu‘ah, because 
    it is very important and it is obligatory to attend it, and one misses out 
    by missing it. So it is a serious matter and is a weekly obligation. 

    To sum up: Jumu‘ah cannot be compared with other prayers, but 
    if he avoids such situations so that he will not be distracted from praying 
    in congregation, that is better. 

    Whatever the case, if his buying and selling may distract him 
    from performing prayer in congregation, then it is haraam, but in some cases 
    the prayer may be delayed, if the Imam is late, so a person may buy or sell 
    something on his way, and it may not affect his attending the prayer. But if 
    he avoids that and adheres to what applies to Jumu‘ah, that will be more on 
    the safe side, so it is like Jumu‘ah with regard to precautions. End quote. 

    From Fataawa Noor ‘ala al-Darb

    And Allah knows best.