Rights of an Unpaid Seller against the Goods
(1) Right of Lien: 
An unpaid seller has a right to retain the goods till he receives the price. But to exercise this lien
(i) He must be in possession of goods
(ii) The goods must have been sold without any stipulation as to credit or where goods have been sold on credit, the terms of credit must have expired.
(iii) It must be remembered that the right of lien depends on actual possession of goods
Where the Property in the Goods has Passed to the Buyer.
(2) Right of Stoppage in Transit: 
Where the seller has delivered the goods to a carrier or other bailee for the purpose of transmission to the buyer, but the buyer has not acquired them, then the seller can stop the goods and regain the possession. 
(3) Right of Resale:
The unpaid seller can resell the goods
(i) Where they are of a perishable nature or
(ii) After exercising his right of lien or stoppage in transit, even though he has given intimation to the buyer of his intention to resell, buyer has not tendered the price within a reasonable time.
(iii) Where the seller has expressly reserved the right of resale in the contract itself.

(1) Existing Goods
(i) Specific Goods - Specific goods denote goods identified and agreed upon at the time of contract of sale.
(ii) Ascertained Goods - (a) The term ascertained goods is also used as similar in meaning to specific goods.
(b) But this term may even refer to goods which become ascertained subsequent to the formation of the contract.
(iii) Unascertained or Generic Goods - These are goods which are not identified and agreed upon at the time of contract of sale.
(2) Future Goods
These are goods which a seller does not possess at the time of contract of sale but which will be manufactured or produced or acquired by him after entering into the contract of sale agreement
(3) Contingent Goods
Contingent goods are the goods, the acquisition of which by the seller depends upon a contingency (an event which may or may not happen). Contingent goods are a part of future goods.

Following essential elements are necessary for a contract of sale
(i) Two Parties :
(1) A contract of sale involves two parties - the seller and the buyer.
(2) The buyer and the seller should be two different persons. If a person buys his own goods, there is no sale.
(ii) Transfer of Property :
(1) To constitute sale, the seller must transfer or agree to transfer the ownership in the good to the buyer.
(2) A mere transfer of possession does not amount to sale.
(iii) Goods :
(1) The subject matter of contract of sale must be goods. It excludes money, actionable claims and immovable property.
(2) The term 'goods' includes every kind of movable property, stocks and shares, growing crops etc.
(3) Goodwill, trademarks, copy rights, patent rights etc., are all also regarded as goods.
(iv) Price :
(1) The monetary consideration for the goods sold is called price.
(2) If goods are exchanged for goods, it is only barter and not a sale.
(3) But if goodsare sold partly for goods and partly for money, the contract is one of sale.
(v) Includes both 'Sale' and 'Agreement to Sell' :
(1) The term contract of sale includes both sale and agreement to sell.
(2) If the property in goods is transferred immediately to the buyer it is called a sale.
(3) On the other hand, if the transfer of property takes place at a future date or on fulfillment of certain conditions, it is called 'an agreement to sell'.