What is the difference between buying an item and purchasing an item?

buying is refer for small products

Purchasing is refer for contracts and big products

Explanation:

buying and purchasing are considered as synonyms and both term that are often used interchangeably, these both terms meaning to acquire something against money

Buying an item is considered general term and very commonly used to refer to everyday goods and buying is refer for small products

example; I bought a new phone

and

Purchasing an item is considered to be more formal word than the buying an item and Purchasing is refer for contracts and big products

example; I purchased a piece of land