What liability is imposed on the seller of goods as regards their merchantable quality and their fitness for the purpose of those goods?

As a general rule, a seller of goods is not liable if the goods or not of merchantable quality or fit for the purpose for which they are obtained.
This is the principle of Caveat emptor. However under section 16 (a) of the Sale of Goods Act, if the buyer expressly or by implication makes known to the seller the particular purpose for which the goods are required, so as to rely on the sellers skill or judgement, there is an implied condition that the goods shall be reasonably fit that particular purpose.
This condition is not implied if the goods are sold under a patent or other trade name.
Under section 16 (b) of the Sale of Goods Act, where goods are bought by description from a person who deals in such goods in the ordinary course of business, there is an implied condition that goods shall be of merchantable quality.
This condition is not implied if the buyer has examined the goods but failed to detect defects which such examination ought to reveal.