The word (sells) is translated from the Arabic word (Shara), which could mean either selling or buying, on the other hand the Arabic (Yashtari) just means (buy). Allah (swt) says:

  {وَشَرَوْهُ بِثَمَنٍ بَخْسٍ دَرَاهِمَ مَعْدُودَةٍ وَكَانُوا فِيهِ مِنَ الزَّاهِدِينَ }

(And they sold him for a reduced price – a few dirhams – and they were, concerning him, of those content with little.)  (Yusuf: 20)

( وَمِنَ النَّاسِ ) (And of the people..): Meaning, some people.

( مَن يَشْرِي نَفْسَهُ ابْتِغَاءَ مَرْضَاتِ اللَّهِ ۗ ) (who sells himself, seeking means to the approval of Allah..): That is, who sacrifices himself in exchange of Allah’s pleasures.

 إِنَّ اللَّهَ اشْتَرَىٰ مِنَ الْمُؤْمِنِينَ أَنفُسَهُمْ وَأَمْوَالَهُم بِأَنَّ لَهُمُ الْجَنَّةَ ۚ يُقَاتِلُونَ فِي سَبِيلِ اللَّهِ فَيَقْتُلُونَ وَيُقْتَلُونَ ۖ وَعْدًا عَلَيْهِ حَقًّا فِي التَّوْرَاةِ وَالْإِنجِيلِ وَالْقُرْآنِ ۚ وَمَنْ أَوْفَىٰ بِعَهْدِهِ مِنَ اللَّهِ ۚ فَاسْتَبْشِرُوا بِبَيْعِكُمُ الَّذِي بَايَعْتُم بِهِ ۚ وَذَٰلِكَ هُوَ الْفَوْزُ الْعَظِيمُ

(Indeed, Allah has purchased from the believers their lives and their properties [in exchange] for that they will have Paradise. They fight in the cause of Allah, so they kill and are killed. [It is] a true promise [binding] upon Him in the Torah and the Gospel and the Qur’an. And who is truer to his covenant than Allah? So rejoice in your transaction which you have contracted. And it is that which is the great attainment.)  (At-Tawba: 111)

Therefore, you find that the martyrs are the ones who sold themselves in the way of Allah’s pleasure.

 وَلَا تَحْسَبَنَّ الَّذِينَ قُتِلُوا فِي سَبِيلِ اللَّهِ أَمْوَاتًا ۚ بَلْ أَحْيَاءٌ عِندَ رَبِّهِمْ يُرْزَقُونَ * فَرِحِينَ بِمَا آتَاهُمُ اللَّهُ مِن فَضْلِهِ وَيَسْتَبْشِرُونَ بِالَّذِينَ لَمْ يَلْحَقُوا بِهِم مِّنْ خَلْفِهِمْ أَلَّا خَوْفٌ عَلَيْهِمْ وَلَا هُمْ يَحْزَنُونَ * يَسْتَبْشِرُونَ بِنِعْمَةٍ مِّنَ اللَّهِ وَفَضْلٍ وَأَنَّ اللَّهَ لَا يُضِيعُ أَجْرَ الْمُؤْمِنِينَ

(And never think of those who have been killed in the cause of Allah as dead. Rather, they are alive with their Lord, receiving provision, % Rejoicing in what Allah has bestowed upon them of His bounty, and they receive good tidings about those [to be martyred] after them who have not yet joined them – that there will be no fear concerning them, nor will they grieve. % They receive good tidings of favor from Allah and bounty and [of the fact] that Allah does not allow the reward of believers to be lost.) (Ali-Imran: 169-171)

( وَاللَّهُ رَءُوفٌ بِالْعِبَادِ ) (And Allah is full of kindness to [His] servants.): The word Ra’uuf (full of kindness) gives a more precise meaning, and the word Rahim (compassionate) gives a broader and more comprehensive meaning. Therefore, out of Allah’s mercy, His compassion and His tender for His creation that He endowed this high status for the righteous martyrs, and that they exceed the “barzakh” stage after their martyrdom, and to be with Allah (swt), so Allah (swt) says:

 {وَلَا تَقُولُوا لِمَن يُقْتَلُ فِي سَبِيلِ اللَّهِ أَمْوَاتٌ ۚ بَلْ أَحْيَاءٌ وَلَٰكِن لَّا تَشْعُرُونَ }

(And do not say about those who are killed in the way of Allah, “They are dead.” Rather, they are alive, but you perceive [it] not.)  (Al-Baqarah: 154)