A summary of the view of the Imaam, the Scholar, Shaikh Muhammad Naasir ad-Deen Al-Albaanee (rahimahullah):
“Zakaah is obligatory upon the two currencies of gold and silver. As for other than these two, such as the other metals, then Zakaah is not payable. The exception being the well-known difference of opinion amongst the scholars regarding the obligation of paying Zakaah upon the merchandise for trade. As for metals, then there is no Zakaah payable upon them due to the absence of textual evidence.
The merchandise that is for the purpose of trading, then it refers everything that a person possesses for the purpose of trading (i.e. to sell) and it does not refer to any specific [type of] product .
The Merchandise for Trading (i.e. For selling): Then this is a category of Zakaah regarding which the Scholars have differed about since the olden times. Some of the scholars considered it an obligation to pay Zakaah upon this category whilst others considered it not to be an obligation.
So when it is said: ‘Is Zakaah to be paid upon the merchandise for trade?’ Then the intent in the question here by the term ‘zakaah’ is the prescribed codified level of Zakaah that is paid conditional upon reaching the Nisaab (the minimum amount after which Zakaah becomes obligatory) and after a year has passed whilst in one’s possession.
So those who consider the ‘zakaah’ to be obligatory, then their intent is that it is obligatory to pay the Zakaah annually – so if the value of the merchandise reaches the Nisaab amount then Zakaah is payable to the value of 2.5% just as it is paid upon gold and silver.
This categorization of Zakaah is not found in the Sunnah nor in the Book of Allaah that supports its obligation.
However, there is a general Zakaah that is obligated upon any person who finds that he has plentiful and extensive trading goods [for sale]. So he pays a non-specific amount that is not conditional upon possession for a year – and this is done in accordance to a large number of general texts related to giving [the Zakaah and charity], such as the saying of Allaah, the Most High: “O you who believe! Spend of that with which We have provided for you.” (Al-Baqarah 2:254)
As for fixing a time and specifying 2.5% of it to be given (as Zakaah), then that has no evidence [to support it] in the authentic Sunnah.
And I say: The authentic Sunnah because there are some narrations in ‘Sunan Abee Dawood’ and in other places that would prove, if they are authentic, the obligation of paying Zakaah upon merchandise that is for trading.” 
Some Narrations and Proofs:
1. Ibn `Umar (radiyallaahu `anhumaa) said:
“There is no wealth that is attained in the trade of slaves, or livestock, or cloth except that there is in that a Zakaah that is payable upon it yearly.” [1*]
2. Ibn `Umar said:
“There is nothing from the possessions [of a person] that Zakaah is payable upon except those for trade.” [2*]
In these two narrations, therefore, it is established that Zakaah is to be paid annually upon goods for trading. That is the Madh-hab of Abu Haneefah , Maalik , Ash-Shaafi’ee  and Ahmad .
Abu `Ubaid (rahimahullaah) narrated from some of the Jurists: “There is no Zakaah payable upon the wealth/possessions of trade” .
However what is correct, and Allaah knows best, is that Zakaah is payable upon goods that are for the purpose of trading so long as the Nisaab is reached equivalent of either gold or silver (i.e. the minimum amount, above which Zakaah is payable). This is because the above narrations from the Sahaabah take the ruling of being ascribed to the Prophet (salallaahu `alaihi wassallam) because the likes of this speech is not uttered with one’s personal opinions.
Furthermore, these narrations were not opposed by any of the other Sahaabah – this is more-so the case when it is known that these saying were widespread – and in the absence of any opposition, it strengthens the case of their consensus upon this ruling .
Abu `Ubaid (rahimahullaah) himself asserted that in his view the Zakaah upon the wealth (or possessions) of the traders must be paid, and upon that is the consensus of the Muslims. The opposing opinion is not from the view of the people of knowledge .
Ibn Mundhir sated: “They (the Scholars) have agreed that Zakaah is payable upon goods that are traded if a year has passed over them.”  Al-Bayhaqee asserted likewise .
Ibn Taymiyyah sated: “As for goods that are for trading, then Zakaah is payable upon them. That is narrated from `Umar, and his son, and Ibn `Abbaas – and the seven famous jurists [9.1], and Al-Hasan, Jaabir Ibn Zayd, Maymoon Ibn Mihraan, Taawoos, An-Nakha’ee, Ath-Thawree, Al-Awzaa’ee, Abu Haneefah, Ahmad, Ishaaq, Abu `Ubaid.
And [the contrary] has been narrated from Maalik and Daawud who said: There is no Zakaah payable.” 
And what is correct is that Maalik agreed with the paying of Zakaah upon goods for trade but differed in some details .
As for Daawud Adh-Dhaahiree (rahimahullaah) and the Dhaahiriyyah (the Literalists), then in this affair their opposition is not considered because they were preceded by the consensus (ijmaa`). Furthermore there is proof in the saying of Allaah: “O you who have believed, give in charity (i.e. Zakaah) from the good things which you have earned and from that which We have produced for you from the earth.” (Al-Baqarah 2:267)
Then there is a discussion amongst the people of knowledge regarding whether the Zakaah is payable upon the goods themselves or the value of the goods.
Ibn Hubayrah (rahimahullaah) said: “The scholars have differed as to whether the Zakaah upon the goods of trade is obligated upon their value or upon the type of merchandise itself. So Abu Haneefah said: ‘It is obligated upon the type of merchandise but the market value of the merchandise is given consideration. So if the value reaches the nisaab level, then a person can give 2.5% of the merchandise or 2.5% of its market value.’
Maalik and Ahmad both said: ‘Zakaah is payable upon the value of the merchandise and not on its type – so it is paid upon its value.’
And Ash-Shaafi’ee said: ‘The obligation is upon the market value – on this there is one saying. So is the Zakaah taken from a portion of the merchandise itself or can it be given in its equivalent value? Upon this there are two sayings.'” 
Notes:[1*] Reported by Ibn Zanjawaih in “Al-Amwāl” (3/943) with an authentic chain of narration – reported by Abu Ubaid in “Al-Amwāl” (p.521). Abdur-Razzaaq in “Al-Musannaf” (3/943) [2*] Authentic chain of narration. Report by As-Shāfi’ī in “Al-Umm” (2/46), Ibn Abī Shaybah in “Al-Musannaf” (3/183), Ibn Zanjawaih in “Al-Amwāl” (3/942), Al-Bayhaqī in “As-Sunan al-Kubrā” (4/147) and in “As-Sunan as-Sughrā” (2/58). Ibn Hazm graded it Saheeh in “Al-Muhallā” (5/34). The verifier of Ibn Zanjawaih’s “Al-Amwāl” declared it Saheeh, and likewise Al-Albānī in “Tamām al-Minnah” (p.364)  “Badaa`i as-Sanaa`i” (2/20), “Al-Ikhtiyaar” (1/112)  “Al-Kaafee” (pp. 96-97), “Al-Fawaakih Ad-Dawaanee” (1/384-385)  “Al-Haawee” (3/282), “Al-Majmoo` Sharh al-Muhadhdhab” (6/47)  “Al-Insaaf” (3/153), “Ar-Rawd an-Nadee” (pp. 151-152)  “Al-Amwaal” of Abu `Ubaid (p. 523)  “Majmoo` al-Fataawaa” (25/15), “At-Tamheed” (17/126,132)  “Al-Amwaal” of Abu `Ubaid (p. 525)  “Al-Ijmaa`” of Ibn Al-Mundhir (p.51)  “Sunan al-Kubraa” (4/147) [9.1] The “Seven Jurists” are from the taabi’een of the city of Madeenah, famous for their great knowledge, virtue and understanding. They are: Sa`eed Ibn Musayyib (d.94H), `Urwah Ibn Zubair, Al-Qaasim Ibn Muhammad Ibn Abee Bakr as-Siddeeq, `Ubaidullaah Ibn `Utbah Ibn Mas`ood, Khaarijah Ibn Zaid Ibn Thaabit, Sulaimaan Ibn Yasaar – then they differed over the seventh. It is said: Abu Salamah Ibn Abdur-Rahmaan Ibn `Awf (and that is the saying of most of the scholars). Others said: Saalim Ibn `Abdillaah Ibn `Umar Ibn Al-Khattaab or: Abu Bakr Abdur-Rahmaan Ibn Al-Haarith Ibn Hishaam Al-Makhzoomee.
إذا قيل من في العلم سبعة أبحر روايتهم ليست عن العلم خارجه
فقل هم عبيد الله عروة قاسم سعيد أبو بكر سليمان خارجه
“At-Tarjeeh Fee Masaa’ilis-Sawm waz-Zakaat” (pp. 132-142) of Shaikh Muhammad Ibn `Umar Ibn Saalim Baazmool.
“Al-Masaa’il Al-`Ilmiyyah wal-Fataawa Ash-Shar`iyyah – Fatawash-Shaikh Al-`Allaamah Muhammad Naasir Ad-Deen Al-Albaanee Fil-Madeenah Wal-Imaaraat.”